There is a scan of the ITT501 KSU installation manual located at
Please provide this type information via PM or email to fellow installers.
Thanks, Bill
(careful -- all the pages are there, but not in order)
The ITT501 is a larger system, but a near-clone of the WE501 and the wire designations and the installation style is the same as the 551, so it should provide good guidance.
Also, there was a guy who sells A-lead adapters periodically on eBay for $20, listed as "A-Lead Control Unit for Western Electric 1A2 Key System". If you are installing traditional 500-style single line phones, some of them can be rearranged to provide A-lead control from the hookswitch. Depends on the model.
