72 C/S factory 455 U code with external regulator. When the alt was rebuilt 20 years ago, I asked him to upgrade it from 55A to 61A (I have also seen 63A?) and the Gen light has been on/dim pretty much ever since. Replaced the voltage reg. a few times for the heck of it way back then but always seemed to be one size fits all.
AC Delco lists both C635 (GM 89058245) and C663 (GM 01119519) for my car and states 37,42,55,61 for both? Plus I am seeing D663 all over the place, not C663 like on the ACDelco site? C for 'capacitor'? I do have a radio and mine does have the capacitor..

About to start systematically testing volts/amps at battery, underload, alt, at reg., etc....

 

The factory parts book shows 1119515 as the correct regulator (unless you have the electric rear window defogger, in which case it is 1119519). 1119515 is a D635.

 

Funny thing, I did add the electric/heated glass! All the correct hardware/electronics swapped over from a late-build 71 supreme I owned simultaneously several years back. might explain the whole Gen light, then again, not exactly sure what regulator I have in there now...
Just found receipt: D663 B ACDelco from Trak Auto 8-15-96. I remember going thru issues a a few reg's but only see one receipt, and this was before I added the heated glass..

 

D663 would be correct.

 

Yup. But I've been seeing a cross over number VR-171 which I believe is Standard's number for the D663? When I search for GM 01119519 I get D663 and VR-171?