Yes, I also have a 1982 serial number Marshall Lead 12, a 10" combo. No output jacks of any kind, two inputs one high one low. And yes, the board is exactly like the one labeled as old board on the post by 5005_Owner. The board is marked as JM73 (?)
Quality is above any other Marshall 12 I've seen. Brushed gold metal front, plywood instead of particle board, knobs with fitting screws, handle with brass/gold platted fittings, and the back panels are also plywood covered with tolex, not just painted cheapboard or particle board.
Celestion G10D-25 stamped frame Made in England inside.
The words/number Lead 12 are printed larger on the front (it doesn't have any output jacks) and serial number is stamped/engraved on the metal just below them.
It has been confirmed as 1982 built by Marshall UK, but they didn't provide any other info. Said that, they offered sending the schematics to a registered repair workshop after it developed a fault in one of the inputs (now solved).
There's a dude in youtube playing one of these:
So yes, all info pointing out that these started to be built around 1984 is wrong. And the one referred as the 'old' model, could well be this, not the one output vs two outputs later model.
It is a wonderful little amp.
D