The two Morning Sun color books on tank cars by James Kincaid are extremely well done and highly recommended. Vol 1 is cars with full center sill/frame. Vol 2 is cars with "stub sills" (no frame, the car body is the frame so to speak).
Author James Kincaid writes "tank cars built without full center sills are certainly nothing new. Van *** had built a number of cars to this design in the late 1880s. However the concept never went anywhere and it was not until the advent of the high capacity tank car around 1962 that the design feature was reintroduced. Most modern tank cars are built this way, with the exception of those constructed with aluminum tanks or the other rare cars ...."
Dave Nelson
OK Dave, I see that on their time line in 1891. I'm going by the limited area of the prototype tank car their web site shows, vs the picture of a model you posted. The walkway and ladder match.
Mike.
My You Tube
The UTLX class V tank car predates that by a couple decades:
Dave H. Painted side goes up. My website : wnbranch.com
According to Union Tank Car Co., about their product timeline, 1954 was the start.
https://www.utlx.com/utlx-history/
About when were frameless tank cars introduced and became commonplace?
Joe Staten Island West