Another timely question, Dan! My #770 just came in the mail today...I wanted it even if it would take some work to fit O gauge, and I really do appreciate the AF accessories, their execution is great.
I was curious about the differences as well. What I found found before shopping is the 770 was 1950-1952, and the 775 is ‘53-‘55. Couldn’t find much about later production, and I couldn’t find any discernible difference other than the black base. The modern era Lionel remake also has a black base.
Unlike the AF sawmill & AF barrel loader, which I had to raise 3/4” to operate, the 770 seems fine as is. The green box car is O27 K-Line, which is actually closer to S than O gauge. The Hormel box car is O, and also appears to be low enough to work. Nothing I own is 1/48 O scale, which I assume would be too big. I think “traditional” size O rolling stock on tubular track should work just fine.