the three types AFAIK:
5002:
a special design, with tertiary feedback and DC balance windings. the core is gapped because the supply current flows through the primary. a fairly complicated design, and optimized for the class A output circuit, similar to the 1108 preamp.
B11148:
when the output circuit changed to a class AB design based on the 1109 preamp, the output transformer changed as well. not gapped; four equal ratio windings, usually wired as a 1:2 stepup. also used as the output transformer in the la3a.
12614:
a simpler design, with a 1:1 ratio, adding no gain but providing a lower output impedance.
all three are EI steel core transformers. as for differences in sound, the change from the 5002 to the B11148 coincided with a complete redesign of the output circuit, so it isn't a fair comparison. the B11148 and 12614 are more similar, but different enough, and with enough minor changes to the 1176 circuit that i doubt that is worth debating either.
ed