Congratulations on that one Frank. It really amazes me how stupid I can be at times. I must have looked at that auction listing 5 or 6 times without the light going on. And I have a copy of OP1664 on the shelf with the line drawing of it too!
The earliest manual I'm aware of on the M20 3.5" launcher is the TM9-297 from December, 1948. The T74 was standardized as the M20 3.5" rocket launcher on October 11, 1945 but the planned production of 5,000 launchers was cancelled as unneeded after the war ended. And, of course, they didn't do too much more in the next five years, so they were caught flat footed when the Korean war started and had to issue old 2.36" bazookas to shoot at the T34s.
Note the rounds by 1948 had the shorter M28 style warhead but still had a unique fin. It is described as being aluminum in the TM and protrudes further forward than later types. Not as unique as the T80/T85 fin, but different than is normally seen. As can be seen the the data table, they were up to the "E2" varient by then. Color and markings codes seem pretty standard and I'll bet the early rocket was similar.
We have an example of the HEAT rocket, in cutaway, at the museum I work with, but I'm pretty sure it has been repainted and won't tell us much. Dittos for the one that used to be on display at Picatinny. The third image below is a specimen in another friend's collection. Perhaps it's a transitional piece as it has the long, early warhead but the later, 1st pattern M28/M29 tail fin. And, unfortunately, no paint. Hope this helps a bit.
