And yes, from what I’ve heard, the extra numbers on the current rollsigns are there in case they add variant services or decide to eliminate some of the confusing splits, like 5 trains that run to two different places in the Bronx or the 7 express and 7 local. Maybe 13 is for something like a Seventh Avenue-Dyre Avenue routing.

Until the “Chrystie St.” changeover in 1967, the BMT trains were primarily referred to by NAMES, not letters or numbers.
On maps and on station signs, the trains were Sea Beach, West End, Brighton, etc. People would use those names when referring to any BMT train in their conversations.

So it was in 1967 that the 14th St.-Canarsie line officially became the LL. Many years later it became the L when all of the double-letter names were abolished.

When the Chrystie Street Connection opened in 1967, everything went over to letters and (for the IRT routes) numbers as the primary route designator. That’s not to say that on old BMT routes you still didn’t see some traincars with their old route numbers now and then.