Attached is the ER1400 original datasheet.
Why they started using battery backed SRAM - the question is simple - SRAM is much faster (200ns (now 70ns) vs 10ms write time) 64 kilobits instead of just 100words x 14bits, and comparable guaranteed data retention of both - 10 years.
Today there is a good replacement ferroelectric RAM chips (FM16W08) for the "B" units with 150 years data retention and 10 trillion write/erase cycles. When this chip is used, the problems with NVRAM are over. So IMHO even today the second solution is a better one. Moreover - ths scope saves not just the calibration constants in it (512 bytes out of 8192), but also the last front panel settings and several front panel setups you can recall at any time. It uses it just like ordinary SRAM - it is on the same address/data bus with other RAM chips on the board.
But anyway - there is a separate thread with much more info for the "B" units.