The Yashica Mat 124G takes a 30mm bayonet type filter. Meaning it mounts with a partial twist and a click. These are also referred to as B30 or Bay-1 filters. Other TLR cameras taking this size include Rolleicords, some Rolleiflexes, and Autocords. There may be others as well. As would be expected, the lens hood is also a bayonet mount.
I own and use one of these cameras. Hope this helps.