GUID and UUID refer to different things in general. Also, depending on the context, they can be longer than 128-bits. Oracle Coherence API defines 256-bit UUIDs for example (which is clearly not RFC 4122 or Microsoft GUID, but it still is an identifying number which can statistically be called unique, which a UUID is).

As long as it meets the statistical properties for collision, I don't see any problems with making a 50-bytes GUID. The essential thing is the statistical property, not the number of bits.

Do you think it's more likely that they invented a unique id scheme that requires 50 bytes, or that they're using an overly wide datatype for ordinary UUIDs?

And I have to pick one of those two options?

Like I mentioned, there are different UUID/GUID schemes using more than 128 bits publicly available already. You don't have to invent one. They may simply be using one of those with additional bytes reserved for future algorithmic changes (using first few bytes as a header to specify the algorithm).

But is it likely that they are? I would imagine no.