I finally found the Reading F7-ABA units I was looking for. Unfortunately after some mild track use, one of the wires in the B unit “plug” that hooks into the A unit slot has a break in it. With the break, the unit runs but noticeably hesitates. I got around it temporarily with some electrical tape but I want a long term fix. These units are old and I can imagine after some heavier use I might expect more wires to fray or break.
It’s probably very unlikely that I’ll be able to find another one of these wiring harnesses anyplace unless I buy another B unit and use its harness.
I plan to take it in to my local guy to see if there’s a fix for it.
The AA units will run fine without the B but the B has the horn.
Ideas welcomed👍