A little bit more info on the magnifier:
It's a strange little device. You can see in the picture that it sticks out quite a bit. I would guess that the magnification is 2x or 3x. You can't see the whole viewfinder when it's down (infront of the eyepiece), just the centre portion of the focus screen (the split prism part on manual focus cameras). To deal with this it can flip up.
It's hard to explain the "won't snap shut"Because it can flip up and down it has a little ball that fits into a hole. This should make it possible for it to lock into the "down" position. However, it doesn't lock into this position. One thing to note though is that when your eye is against the magnifier it holds it down for you (so I don't think its a big deal that it doesn't lock/snap, but that's for you to decide, not me).
I know it fits spotmatics and my Pentax k2. I'm not sure about anything else (so I don't know about your cameras.
A quick search and it shows up here: http://www.bdimitrov.de/kmp/misc/focus/index.html under magnifiers. This says 2x. Its not the one in the magnifier-M or magnifier-FB picture though.
The first link under magnifiers ( http://www.bdimitrov.de/kmp/misc/focus/magnifier.jpg ) shows it in the flipped up position.
F3's non-firing issue without the drive might be a loose wire on the battery box, if nothing turns on with batteries in the box. Had that happen with my last F3.
APUG: F4, F3P, F2ASx2, F, FM2n, Nikomat FTn - all blk bodies; Nikomat FT2 chrome
Nikkors: 18-70/3.5-4.5G AF-S DX (for DPUG), 28/3.5 H, 35/2 O, 50/2 H x2, 50/1.4 S, 55/2.8 Micro AIS, 85/1.8 K, 200/4 AI, 300/4.5 EDIF AIS
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