Thought I would add some information I've discovered recently while playing with these idler arms:
There are 2 different Ford engineering numbers cast into the arms being discussed;
C7ZA-3355-B = Mustang arm, both manual and power arms - each with different size bores on the center link bushing end.
C7OA-3355-B = Fairlane/Torino/Falcon arm both manual and power - same bore difference but about 1/4" shorter than the Mustang arm.
Service replacement arms without cast eng. numbers:
C7ZA arms have a faint 283 number on the arm - all else dimensionally the same as the factory arm.
C7OA arms have a faint 183 number on the arm - looks same as the 283 but again is about 1/4" shorter.
This info is from carefully examining NOS replacement arms of both types, and comparing to factory pieces.
At first glance, the 183 and 283 can appear to be 188 and 288 but the last number is a 3.
I believe that TRW was one manufacturer for service parts - if you can find a vintage TRW part # 18686 with the nut and crimped threads it will be virtually identical to the C7ZZ-3350-A service replacement power steering arm (without eng # but with 283#). TRW 18687 is for manual steering.
Vintage Perfect Circle part # 268-3497 and 268-3498 for power and manual arms also have the 283 on the arm itself, as if from same source.
Some, if not all of the Perfect Circle arms had a 3 hole bracket, even for power steering - which is confusing. Even though it had 3 holes, the other dimensions (length, etc) same as 2 hole bracket.
I think there is a Perfect circle 268-3498 manual arm now on EBay. for reference.
Well, that's about all I have - again, this is just my personal experience and thought I would add it to the thread.