NMP on front slam plate? (NMP in a diamond shaped emblem) = Nuffield Metal Pressings, (The plant that stamped the panels out, part of BMC but seperate from Austin, Morris etc)
Small plate on bulkhead stamped with numbers? = Body number tag. (Somtimes you have a body number as well as a VIN number quoted on Heritage certificates etc. This is that one).
Plates similar to FE plate on both front floorpans? = Floor Number. Identifies that body section in the factory, just like an FE number only for a different sub assembly.
And a small bolt with a welded flat top going through front floor pans (both sides)? = This is a fixture to which the car was attached in the factory. There are also sometimes 4 similar ones one in each corner.