Bristol 409 - engine numbers

[Peter Valvona]

I've got a 409 that doesn't have the original engine and I'm trying to identify where the replacement came from.

The funny thing is that the engine number is only about 250 further on from the original, so I'm wondering if a later engine was fitted at Bristol cars.

The engine has "scalloped" valve covers which suggests it's a Polyspherical 318 engine ........... does anyone else have one of these?

Is the stamped in number on the RH side of the engine block (at the front, just below the cylinder head) a Bristol number or a GM number?

Does anyone else have an engine number that's something like 1641 - 111***C?