The 4th procedure says with AVC on and V4 replaced, adj meter sensitivity with SENS pot R23. (?) What am I using for a reference to adjust the sensitivity?
Good question and the answer will not be nearly as good.
What you use to adjust the sensitivity pot is largely subjective although there has been something of a standard for years. Part of the problem with receivers like this is that the S-meter response will tend to vary from band to band and there is not much you can do about it.
However, if you would like a standard signal to work with the answer would be a 50 µV carrier to produce an S-9 signal. Now the question becomes how do you get a 50 µV signal. Well that kind of depends on the generator. 50 µV from a 50 Ohm generator would be -73 dBm output into a 50 ohm load. Some generators are calibrated to do such things (typically modern and expensive), some are not (most vintage generators).
The alternative is to produce or find a signal that you consider to be "good" but not overwhelmingly strong and set the sensitivity pot so that this signal represents S-9. Typically this would be a signal that is strong enough that you hear very little background static or hiss (i.e., a nice clear signal).
You can tweak that sensitivity pot until you are satisfied that a good signal shows up as around S-9 and a really really really strong signal pushes the upper end of the meter. Then a weak signal sould be down in the noise wherever that might be (typically 0 to S-5, but sometimes even higher depending upon you local noise sources).
Everything seems relative except for V2 RF 6BA6 pin 5. Aim point is 505 but my reading shows 164. (Typo?)
I guarantee that 505 is not correct. The maximum tube rating for a 6BA6 is 330 and they are typically used between 100 and 250. I would venture that what you have is just fine.
By the way, if you ever want AVC action while you are listening to SSB stations here are some audio derived AVC circuits that could be added as a modification without any real change to the receiver itself (removing a ground connection at the Man-AVC-BFO switch, then attaching a circuit to the top of the volume control, 105 V line, ground and the AVC line at the Man-AVC-BFO switch. http://curtiseickerman.weebly.com/technical.html
The circuit was used with an HQ-129X, but the HQ-140X is the same receiver design just with the miniature tubes. With this AGC I found a more extensive product detector modification to be unnecessary.
I used the tube version of the Audio Hang AGC for years. I never got to try the solid state versions that could easily be contained under the chassis.