With out seeing a picture commenting is not easy nor do I expect it to be perfect but lets try
IMHO what you found is pretty typical if I understand correctly
I cleaned the axle housing and 3rd member today. (05/05/67 Dearborn, 8" 2.79 WCZ-V1). I found a white daub near the fill plug,
On cars with rear fills and even front fills we often find a mark in yellow or sometimes white that many believe indicates that the unit was filled. Seems yours possibly fits into that description
a yellow at the U-joint flange ...
There is a TSB article dated during 66 production describing a yellow spray mark that was applied to the yoke (described to identify one of the two ears). The idea was that Ford was having problems with vibrations and the marking allowed service personnel the ability to remove the driveline and reinstall it with the original orientation. In turn eliminating one possible causes of a drivetrain vibration. The yellow is fairly typically IME for 66 and 67 cars
a blue marking RH side on the 3rd member. The yellow and white markings makes sens to me, but I can not explain the the blue ones on the 3rd member.
By "right" I'm guessing your describing the passenger side face of the third member. What this may be is either what is left of the original writing or (seen many times) drips or runs of marking paint made during the writing.
We often find in one of about three different colors there a number - normally two digits that we think was an assemblers number. Often there were two of these sets of numbers on each subassembly (third member) so this is likely two different people that either had a part in the assembly or an inspector also.
Hope this helps