Moander, if your old Marshall is a top cab with G12M-25s it will probably be a 1960A. In the late 60s and early 70s, 1960s came with 25s, 1982s with G12H-30s - well usually, anyway... Marshall sometimes made different combinations for various reasons, like Celestion running out of some speaker types (Marshall were by far their largest customer in the early 70s), and you sometimes find things that 'shouldn't be'. The top angled cab is the A, the bottom straight one is the B. They do sound different - the A is more complex and slightly brighter, the B is clearer and deeper. There are also two different cone types for each speaker model, with 75Hz or 55Hz bass resonance - the 75s were supposed to be used for lead and the 55s for bass, but again you can find 'wrong' combinations in some cabs.
If you're interested, you can find out exactly which speakers you have and what their date codes are by looking at the ink stamps on the speaker frames, which if they're original will date the cab accurately too.
It really doesn't matter much though, they all sound fantastic! You're right that the plywood they used back then was a lot heavier than the new stuff too, you can easily feel it when you lift the cab. Don't ever sell that cab! Or if you do, find out just how much they're worth now, and don't allow anyone to talk you down, they will never be any less expensive than they are already...