Bum:
I believe that we have a slight communication problem. What you are calling a truck attached with a “dimpled” rivet is what I was calling a “Staple-end” truck which is the term collector reference books use and what you are calling a “convex rounded” rivet truck is what is called a “Bar-end truck”.
Lionel used staple-end trucks from 1946 through 1951 on 4-wheel truck freight cars. Then in 1952 Lionel changed to bar-end trucks on its 4-wheel trucks used on freight cars. The 2257 caboose came out in 1947 with staple-end trucks. When the 6457 came out in 1949 it had staple-end trucks as did the 1950 and 1951 versions. But in 1952 Lionel was using bar-end trucks so the 1952 version had bar-end trucks. So a 6457 can correctly have either staple-end trucks or bar-end trucks. I have three different variations of the 6457’s and two have staple-end trucks and one has bar-end trucks. If you need a photo let me know.
But the 2257 caboose was only produced in 1947 and therefore it only came with staple-end trucks so as Norton and Chuck agreed your 2257 has been modified with a 6457 chassis and because of the bar-end trucks it’s from a 1952 version of the 6457.
Regarding your last set of photos, of my three 6457’s, the two with staple-end trucks do not have those holes between the wheels and my 6457 with bar-end trucks does have those holes, so I do have them both ways.
Hope that clears up any confusion,
Bill