[Eruditass]

I've seen a lot of craigslist items where they say a lens is screw-type but they don't know what it fits and don't have a camera. Now it is a bit of a drive - are there other kinds of screw-type mounts other than M42 and M39's (I assume there are adapters for this and K-mount too)

[wildlifephotog]

There is the T mount. It is a 42mm, but with a different pitch to the threads. T mount adapters are available all over the internet.
I don't know of a K to M39. But there is a M39 to M42 adapter ring.

[ovim]

Theres T-mount which has 42x0.75mm thread if there is no adapter attached. I think this can easily be mistaken for a normal M42 by un "untrained eye".

edit: wildlifephotog beat me to it. Theres also a Leica M39 which has much shorter register distance than the other M39. Heres a nice page about different mounts:
http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html

[Eruditass]

Do you have any pictures that can show the differences?

[wildlifephotog]

T mount and a regular M42 look the same to most. The best way is to see if it screws into a M42 adapter. If it does easily, it's M42. If after a couple a turns it starts to tighten, it's T mount.

[Quicksand]

There's also Leica thread mount (sometimes abbreviated LTM) which is an M39 x 1/26". This looks like the Zenit M39 x 1mm thread, but it's not quite the same, and the Leica version is for rangefinders and not SLRs, so the register distance is MUCH shorter.

When you encounter something labeled "M39" on eBay, use caution!

There are other even more obscure thread mounts, for enlarger lenses, TV camera lenses, CCTV lenses, etc., but I don't know much about those. See here: Link

There are 0.625", 0.8", 24mm, 40mm, 44mm threaded mounts listed there, and I have no doubt others exist.

[xjjohnno]

Didn't very early Asahi Pentax lenses come in M37?