not really, it's been over a decade since i saw a 51 and it was an upgrade from a 21.. that was a pain. i worked on old nt's for years and then went to rolm, came back to nortel after the 81c was the "big" switch.. i was trained at one time (memory block) that if you have a cni then your looking at the c model.. but at that time they still made the 21 51 51c 61 61c 71 71c and 81,81c if i remember right they stopped the 21 and came out with the 11,, 90 percent of the switches ive installed were 61c or 81c's with a few (20) opt 11's..
john poole
bellsouth business
columbia,sc