PC11 & 11/20 Questions

Well, I've now acquired a PC05 high-speed paper tape reader/punch for the PDP-11/20 I'm currently restoring. I still lack the PC11 board, though. Megan's list says the following: M781 PC11 U PC11 control board for KA11 M7810 PC11 U Paper tape reader/punch controller Do I *have* to use the M781 version, or can I also use the M7810 if it's all that I can find? What is the actual difference between the two boards? Next: why were the backplane modules mounted at the top of the BA11, wire wrap up, in the 11/20? It's easy to flip the thing over while it's not in a rack, but when it's in a rack... Some trivia: the two partial 11/20's from which this one is being built have serial numbers 786 and 1102. -- Jeffrey S. Sharp jss(a)subatomix.com