IIRC it also introduced index based addressing. Before that, people just wrote self modifying code to index arrays...
The 8 index registers ("B-tube") are called B0 to B7 in this manual.
The B0 index register is hardwired to have the read-only value "0".
A read-only null register continues to be used in many modern instruction sets, e.g. in POWER or in Aarch64.