The last two Borderers I bought in 1975 would have been on an “N” but I reg’d them on the “P”, both had CU 220 engines , one was 6:600/ 2 speed and the other was RTO 610/D85 axle. From memory neither had PAS although one of them may have had. With regard to buying and storing new chassis it was during a time when Capital allowances could be claimed against profits. But well established operators who had reached, and were content, at a certain size and were making profit it was an efficient way to reduce taxation and have a new motor or two or three on hand to put into service when required for operational purposes or when times were a bit “harder” ! and of course in those days you couldn’t always get your hands on new tackle sometimes without a long wait ! I always recall being in Harry Parkinson’s workshop at Walton -le-Dale in the early 70’s and he had a new 8LXB Borderer stood in the corner 
. I can never recall seeing an “N” reg Sed/Atk 400 I always thought they came out on the “P” reg same as the B Series ERF. But probably I will stand corrected 
Cheers Dennis.