There are some differences between the versions of the 400 KTU line card depending upon the manufacturer. The 400H is the last one made by Western Electric and has less features than previous versions and was a complete redesign since it used triacs and IC chips. It only had 2 reed relays. When you go off-hook on a line using a 400H, you could not put the line on HOLD unless you paused for 2 seconds or the HOLD would not work. I liked the WECO 400G the best. WECO did make a 400E in 1970 but the production was limited for BTL evaluation only. It was the first to have IC chip control circuitry. The WECO 400D had numerous series versions. Bell Labs kept coming up with changes, modifications and fixes to solve field problems that kept poping up. That is why they had so many. The ITT K400D was much more stable and the AECO 1400 series always seem to be going bad. One specific thing about both WECO 400D and K400D, is that the power supply needed an earth ground or the ringup ckt would not work. This was true for all KTU line cards of that design. The ITT K400E was a very good design. It is extremely flexible with all its options although the ring timeout was the same as the K400D, you could add a resistor to make it the same as the WECO 400G and 400H.
There are some sources that still sell 1A2 equipment, I will add another post to this thread after I check to see who still does on a list I have.
When you are servicing 1A2, you can swap line cards to troubleshoot, but you should verify the strapping block, etc to make sure of the options selected when doing so. Most of the time this is not an issue, but it can be if the customer has something special on one line.
Hope this helps!
....JIM....