It does not have "12KB" of ram. It has 2K of 48 bit words. Each word can contain 2 instructions. It is a decimal machine so each word can contain 12 digits. It also has (only) one accumulator (of 12 digits). Whilst this particular example has an indexing mechanism (actually a "pre-modify" instruction) which was added from a 1302, the standard way of indexing is by using "live code" i.e. doing arithmetic on the code on fly. The "2 instructions per word" structure actually makes this very easy to do.
It was designed in the late 1950s and built in 1961. One can program it in machine code (very easy instruction set), an assembler, various obsolete specialist languages and, of course, COBOL. There is no C compiler.
Flossie is very musical. She can play many tunes.