M837
Gives you extended memory control, up to 8 "pages" for a total of 32K of
core. It isn't required to operate the machine but without it you are
limited to 4K of core.
Just to be nit-picky, I'll point out that what this gives you is
8 "fields" of 4KW each. On the PDP-8, a page is much smaller; 128 words
IIRC.