I recently bought another IBM Model M (1395660) on eBay the other day, not thinking that it is actually a terminal keyboard. I thought it has a detachable SDL cable like my current Model M (1391401). I was wrong. Turns out it has a non-removable RJ-45 plug instead, thus being a terminal keyboard. Well, aftering some researching, some were able to get it to work by using an ATMega. I was able to find a pinout of the RJ45 plug on the keyboard, and simply just wire it up to a PS/2 cable. Though apparently, the colour of wire for PS/2 is not standardized, which led to testing each pin to see what it does. I cannibalized an old Microsoft keyboard for the cable and was able to hook the wires up straight to each other.
PS/2 cable wires on a breadboard, original connector above.