Alrightly! I did not understand how attachments work on this website, so here I try again to attach photos (my wife also thinks I can be trained, but the supporting evidence is minimal). Here is another photo of the back side of this amp with the in/out jacks in question. The amp (as I purchased it) had the typewritten labels on it that you can see in the photos. The writing in Sharpie was me ensuring that I could find the right jacks (for the speakers, etc.) in the near dark, which was consistent with the bars and cow barns that I played at in the 1970's.
The line out jack is certainly there, and that is consistent with the first gen schematics that I have, but there is also the input/output jacks near the speaker jacks.
I obtained this beastie used in the mid 1970's, so it could be an early prototype/production model, but I have no way of telling.
Now that I am in my late 60's, and between BMI/ASCAP and Covid19, there are no longer places for local bands to play, therefore I am prepping this amp for sale (along with 5 cabs and 5 other tube amps) and I want my description for Ebay to be accurate. (For some obscure reason my wife does not think big Sunn amps are suitable decorations for our living room. I disagree......if she is out of earshot.)
Later today I will modify my Model T 1st Gen schematics to include these jacks.