So, I ended up with a Koss HV/1 with a bad driver, and an HV/1A with a bad driver. Some differences of note:
The HV/1 has black plastic, and a shiny chrome ring between the ear cup halves. The head size adjusters are glued together. The drivers are bolted together, and have posts to which wires are clipped. The driver magnet is flat. The cups can be removed from the metal band without disassembly - the fingers just pull right out of the sides of the ear cups. There are metal tabs to keep the ear cups from rotating too far. There are strain relief tabs molded into the ear cups for the wires that go through the band.
The HV/1A has a slightly brown colored plastic and cord, and a matte silver ring between the ear cup halves. The head size adjusters are riveted together with two rivets. The drivers are melted together. They're inserted over plastic posts, and then a hot tool melts the post and the driver frame together. The driver magnet protrudes into the rounded area you can see on the ear cup side, with the mylar cone over top of it of course. (The HV/1 driver magnet is flat, and the rounded area is largely empty, just a little mylar bubble there). The cups cannot be removed from the band without disassembly -- there are metal retainers on the fingers that keep them from being pulled out of the drivers. There are plastic tabs to keep the ear pieces from rotating too far. The ear pieces on the HV/1A can rotate 90 degrees so that the headphones can lie relatively flat. (This can't be done with the HV/1s). There is no plastic strain relief molded into the ear cups. Oh and the drivers don't have posts, they've got standard solder pads.
Sound-wise, I cannot tell any difference between the HV/1 and HV/1A drivers. I had a bad driver in each, so I combined them into one good headphone, and with a mono source both sides sound identical. So that's nice. The drivers in both of these suffer from the mylar cone coming apart from the driver itself. There is a sealing ring, and the glue just dries out. This will give you a buzz or a lack of highs. Be careful when disassembling, as the mylar and voice coil will tend to stick in the ear cup, and the magnet back will fall right off, breaking the voice coil wires. I could have repaired the one if that hadn't happened to me. The HV/1A drivers cannot be repaired without a lot of work and care. You'll need a hobby saw to cut them apart, and then you'll need to drill and tap for screws to reassemble it. (or just use glue, but it would be nice to leave these as repairable in the future).
Neither of the HV headphones I own have volume potentiometers. I'm not sure if that was an option for a different year, a different model, or for the International market.
Either one of these are good headphones. The HV/1s have more strain relief and more shiny parts. The drivers are more easily repaired. The HV/1As can lie flat, and have head-adjusters that you can disassemble and repair and re-rivet. (In both cases, the HV/1s have broken plastic tabs in the head adjusters, making them loose and floppy. This can be repaired with a strong closed-cell foam, like a piece of a Mr. Clean magic eraser, glued into the inside of the adjuster to make it have some tension). In all cases, every bit of cloth and foam inside these headphones was rotted away, and needed to be replaced.
These look great with blue and red scrim cloth -- I just used speaker grill cloth, and a little contact adhesive, then some foam behind the cloth.
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Charles.
