I am in search of a seat latch for a 1975 125 enduro with the number 2635 key, it doesn’t need to have a key I just need the latch and lock. 

Welcome to the forum !
And here ya Go !

www.partzilla.com/product/yamaha/1K8-247...bdf8a22a2eb543645a81

Thanks, how do I find out if my key will work with the lock before I order it.

Is keeping the original key important to you?

Repro keys (without the Yamaha logo) can be cut for about $10 - just quote the number on top of the barrel to the key cutter.

With a certain amount of research, it is possible to find out which profile key Yamaha used with which batch of barrel numbers and these each have their own part number and are often available on fleabay as "new old stock" - Partzilla and CMSNL might also have some.

Some clever folks (DEET might be one of them) can re-align the levers within a barrel to fit a specific key. However, I'm not sure if the profile can be changed, so this procedure might have its limitations. Also, the number on the barrel will no longer match the levers sequence. 

Me? I'd just buy a replacement seat latch and live with two keys (or three if I have a locking petrol cap as well!).

I was hoping to have just one key, because the bike is all original and the only piece I’m missing is the seat latch.

Fair enough, and I sort of get that.

If no-one comes up with the goods here, then all that I can suggest is to keep looking on fleabay for a lock/latch with the number that you need - this will, of course, involve lots of patience and leg (eye?) work!

Don't forget to check on Partzilla and CMSNL for the other models that shared this part (more than 30) and use these as a point of reference when searching as well. Also look in "miscellaneous" and "job lot" listings for these models.

Thanks for the help.

Locksmith can key them all to one key.