Red Mountain (1930s-40s) are the ones marked with number and pattern letter(s) only. Century (1950s-60s) are the ones marked "No. X" and "NN-n/n IN.". But we're seeing "Red Mountain" type markings on thicker, small pour spout pieces we would normally attribute to later "Century" period production.
The number inset on the handle could be a few things. On small pieces, it's usually a gang mold number, to identify which one of multiple, connected patterns the individual piece was cast from.
Since it's incised, that means it was on the pattern before the mold was made, so it must be some kind of identifier for some aspect of quality control. On the Century piece, it would make sense that it's a pattern identifier, since, at least on yours, there is no other unique character. On the (what we assume to be) Red Mountain above, however, the C identifies the pattern, so what the 1 signifies there is up for debate. A possibility is that, given that BSR was such a large producer in its day, that several working patterns were made using the 5C pattern as a master, and the small numerals were how they differentiated them.