PDP-11 memory question

I have a PDP 11/23 (M8186 CPU with floating point and MMU options) in a 4 slot BA11-MA box that at some point in its life was an 11/03. It had an M8044-DF 32k memory module, which I'm trying to replace with a 128k M8059-KJ. The system works fine with the 32k module, but won't do anything with the 128k module. I'm not familiar with PDP-11's, but it seems like my backplane is only 18-bit, while the new memory module is 22-bit. I've also read that the M8186 board is only 22-bit compatible after revision C. I can't find any mark on the board showing what revision it is. Is there another way to tell? Also, is possible to modify the 18-bit bus and make it 22-bit, or maybe by swapping out the backplane? I also noticed that on some used PDP-11 web sites that the BA11-M sells for much more than the BA11-S, which I thought had 5 more slots. Is there some reason for this? Thanks, Tom