interchange BMUs boards on Fanuc 6M

hi there
Currently I'm using BMU 64-2 A87L-0001-0016 05E and would like to get spare boards in case my current board goes off. Its hard to get the same coded board but i found others selling different BMUs listed below. Would it be possible to interchange/replace those BMUs to the Fanuc 6M motherboard ?

BMU 256-4 A87L-0001-0018 08G
BMU 1M-1 A87L-0001-0084 05B
BMU 256-1 A87L-0001-0017 05I

Thanks in advance

These are just bigger, but the same old bubble memory. Check also prices for CMOS replacements, should be more reliable - there are Fanuc made, but also third parties, I remember memex.ca