Sounds like a Bay 1, in which case you're in luck. Bay 1 (aka B30) is the standard filter mount for Rollei and Yashica TLR's. They're pretty easy to get hold of, but often cost more than they realistically should - walk away from anything more than a couple of pounts and you'll find another for a better price.
Originally Posted by David Ruby
There's a pic of a Bay 1 on my website - follow the link below, and go to articles -> Filters. It's written about the baby tlr's but they use the same filters as the larger ones.