Hmmm. My B-engine doesn't even have the engine number there. I wonder why I posted that in 2009? Perhaps that place is where it is on later engines because that FSM is for August 1988 onwards.
Here's where it actually is on my 1979 B engine:
It is hard to find and you have to look down on the engine block between the "branches" of the inlet manifold and you may need to scrape away some dirt there to see anything.
(I'll have another look in a minute and edit in the exact details of where it is.)
Edit: Just studied EXACTLY where it is on my B-engine. It is stamped into a machined upward-facing surface on the left-hand side of the cylinder BLOCK that is viewable only by peering downwards between the inlet manifold branches leading to the inlet ports for numbers 3 and 4 (towards the rear) cylinders. (And on my engine that is alongside the dipstick tube.)