I have an old Peugeot and need to replace the cassette, but I'm not sure exactly which one will fit. I've taken the old cassette to several LBS's and they all say nobody makes them anymore. One told me it was a Shimano 600ex. It's a 7-speed cassette with the following gearing: 13-14-15-17-19-21-23

I'm not sure of the year of the bike - I'm guessing somewhere in the 80's. It's got a Reynolds 501 frame, downtube shifters, Campagnolo 700 wheels, Shimano 105 components.

I found a cassette that looks similar at loosescrews.com but it says it's for Shimano UG - and I'm not sure if that's what I have or not. Is UG the same as 600ex? This is all new to me, obviously. The gearing on that one is slightly different but would be an improvement, I think.

Here's the loosescrews.com cassette:
https://www.loosescrews.com/index.cgi...d=931227318491

And here is the bike, freehub and cassette. The tag on the hub in the 3rd photo says "Shimano 105":

Chuckk - many thanks for that link. Great information there. It seems I definitely have a Uniglide, but I'm wondering now if the conversion to Hyperglide referred to on that page may be the way to go.

Thanks again!

The cassette in question in a Uniglide, which as you would have found out, is held on by the last cog, which is threaded.
The hub is a Uniglide-only hub; you will notice that there is no provision for a Hyperglide threaded lockring in the freehub body.
My preferred option would be to upgrade the freehub body to a 7 speed Hyperglide variety, which would allow for a better choice of cassette, as HG cassettes are easier to come by than UG. The freehub body is removed with a 10mm Allen key from inside the hub. You do this by removing the axle assembly, which then allows access to the internals. The only problem you may encounter when doing a swap is incompatibility between the new freehub body's bearing race and the existing axle's cone. However, more often than not it works, so it is worth the effort. I don't know what you mechanical abilities are in terms of servicing hubs etc., but Sheldon' Brown's site is a good reference if you get confused.
If this seems too difficult, then obviously just buying the UG cassette from Loose Screws is a better option. In either case, make sure you get a chain to go with it.

It's possibly more complicated than that -- if the rear hub doesn't have the enlarged "knuckle" on the hub barrel side of the flange, then it's a first-generation freehub; which is not compatible with any of the later freehub bodies. But seeing as how the bike's got a "New 600 EX" (post-Arabesque) crank, maybe it's OK.

Charles - yes the hub does have that enlarged part - where the drive side is a larger diameter than the rest of the hub. But now I'm leaning back toward just getting the cassette anyway, because I'm likely going to sell this bike since it doesn't fit me well.

Antipodes - thanks for the info. I already have the chain - in fact that's what started all of this (new chain on worn cassette = skipping).

Thanks everyone for your replies.

Look, one thing that (I think) you can do, if your smallest (thread-on) cog is OK, is get a HG cassette, take it apart, and reduce the size of the largest projecting tab on each of the cogs with a Dremel grinder. I think that Sheldon Brown mentions this on his freehub page. Then the HG cogs will fit on the UG freehub body. OTOH, if your freehub body has threads on both the outside and the inside of the outer lip -- some do, can't see yours -- then you can also use the smallest cog from the new HG cassette, and thread it onto the interior threads.

The OP's freehub body has a black plastic dust shield flush with the end. This means it can't have threads on the inside.

Well, Loosescrews.com confirmed that the cassette in the link on my initial post will fit this freehub, so I went ahead and ordered that. Seems the path of least resistance. If I was going to keep the bike I'd likely spend the extra money for the HG conversion, but its geometry has never felt right to me. Now I just need to figure out what it's worth.