he'* looking for a pontiac block ID. The site you listed just has Chevy block IDs.
Either way, i did a quick search for his casting number on there and did not turn anything up.
I am wondering if maybe the block was re-tagged. Most of the ones I have seen hafe had the YF code under the casting number, and not over it.
It could also be a casting code specific to the Limo'* since they were produced in so few numbers, maybe the code is not widely known.
Maybe you can see about getting the PHS documentation for the car if available and see.
There are also other places to identify what motor it is, but some require you to get to the short block.
Check here for that information:
Also hit next at the bottom as the 2nd page has more info.