Here's a pic of Jimi Hendrix playing what appears to be an Acoustic Black Widow.  On closer inspection you can see this guitar is a semi-hollowbody and was likely made by Bartell of California, who were the first manufacturers of Black Widows for Acoustic.  Unfortunately, it's impossible to tell if the guitar is branded Acoustic or Bartell.  Reportedly, Jimi also had a similar fretless guitar which was stolen.  A 2nd fretless unit was being built for him when he died and this instrument may be the one that was owned by Frank Zappa.  These would've also been Bartells since the 1st production change didn't occur until 1971 or 1972.

Noteworthy details on this guitar are the f-hole on the bass side (there may be one on the treble side that is hidden by his hand and shadows), volume controls farther back from the bridge, and a strange metal piece just in front of the tailpiece which, thanks to Devin, we now know to possibly be a stabilization or sustain block which likely had the serial number stamped into it.

The 2nd pic is from a guitar magazine and shows an Acoustic Black Widow guitar that was owned by Hendrix and given by him to another musician.  This instrument is undoubtedly a Bartell, as evidenced by the 22 fret neck, long headstock, and the lack of a notch in the bridge to access the 3rd neck screw.

Harvey Gerst on meeting Hendrix and Hendrix meeting the Black Widow:

I got a call on Saturday morning from Steve Marks, the president of Acoustic, saying, "Go down to TGG right now!" Why? "Jimi Hendrix just bought four of our amps and he's having trouble with some of the controls." I'd seen Jimi, but had never had the chance to talk to him. I showed him what he wanted to know about the amp. Then I said, "Here's another thing they can do. Let me borrow your guitar." We're both left-handed. I go to show him this feature, and his strings are like, medium gauge. I'm used to extra super slinky. And they're way up from the fret-board. I hand it back to him and say, "I can't play this. Let me go get mine." Mine was the Acoustic Black Widow, [with] twenty-four frets to the body, so you can play two full octaves. The heel doesn't start until after the 24th fret. One of the techniques Hendrix used was to shut the volume knob down, hit the chord and then bring up the volume. Your finger sat right there. And the toggle switch was angled to hit your strum. Neat guitar! I walk in, take it out and show him the neat little thing. He's not even listening. He's just staring at my guitar. It's left-handed. He's got nothing but right-handed Strats. When I finished showing him some things, he just held out his hands and said, "Gimme." He starts playing that thing, starts tape rolling, and I'm going, "Why do I even play guitar?" He finishes playing. I answered all his amp questions, and said, "I gotta go. Can I have my guitar back?" He says, "My guitar." I say, "No, that's my guitar." He starts laughing and says, "Not anymore! Now it's my guitar!" I say, "Hey look, I'm not a salesman. I play in a band, Sweetwater. What am I gonna use?" He points at the floor where four of his white Strats are laid out and says, "Help yourself!" Of course I wanted another Black Widow. I called Steve Marks and told him what happened. He said, "He wants it? Give it to him! Don't worry. You'll get one back!" I take one of Jimi's Strats to use. Like a dummy, I didn't have him sign it. I'm gonna play this guitar on the road, and I hate it! The action's up way too high. The damn knobs are getting under my arms because it's upside down. Every time I strum I hit the volume knob. Can't keep the damn thing in tune. I put super slinky strings on, and it made the bridge even worse, and now the action's even higher. I'm having to intonate this damn thing every night! I go through a set of strings in one playing. Finally, a month later my Black Widow comes in. I ran into Jimi in the airport in Oakland, and asked him "How are you liking the Black Widow?" He says, "I love it! It's my studio guitar." I said, "Well, I finally got my new Black Widow in, so here's your Strat back." He stared at me for a second, then said, "Huh?" I said, "Here's your Strat. I don't need it anymore." "Oh. Okay." It wasn't till a few years later I was telling this story that it hit me. He wasn't EXPECTING me to give the Strat back. He was GIVING it to me! And I, like an idiot, gave it back to him-one of the first four Strats he owned. What's it worth today? About $500,000. While I had it, Sweetwater was playing a theater that had no sound system. I had an Acoustic cabinet and a powered unit, so we'd use that for a PA. That left me without a guitar amp. Phil Fry of Fry Music, an Acoustic dealer in Phoenix, opens up his store after hours and says, "Pick an amp." There in the corner is a gorgeous Marshall stack with eight 12 [inch speakers]. "That one." I get up on stage, and I'm thinking, "I've got a Marshall stack and I'm playing Hendrix's guitar." At one point I'm supposed to come in and hit this note just full out and hold it and let it feedback. I walk over to the Marshall and crank every knob to eleven! The song starts off with vocals, bass and keyboards. I'm thinking, "This is SO cool!" Comes the point I step forward a little bit and I crank the knob wide open, hit the chord, and it goes "dink". Everybody's looking at me like, "dink?" I hit it again and "dink". It was totally clean! The Acoustic amps I usually played through all had built-in fuzz tone. Hendrix used two fuzz pedals wired in series. The Marshalls didn't have a bunch of gain even at full volume. The later ones did, after they put in the gain control. I must have had the funniest look on my face. I'm ramming the guitar into the cabinet trying to get some feedback, and with all my gyrations, I musta looked like Hendrix! No sound is coming out! I think that's when I got really soured on Hendrix's guitar. I was so embarrassed.


Taken from an interview with Tape Op.  Read the whole interview here:

http://www.tapeop.com/articles/bonus/harvey-and-alex-gerst/

 

Interestingly, Harvey says that his BW had a 2 octave neck (24 frets).  At this point, the BW was likely a Bartell, which usually had 22 frets (see the Hohner BW