Some maybe all of the 261 truck engines had exhaust valve rotators (a worthy addition in my opinion). Mine came from a bone yard in Wisconsin as a standard bore replacement block with no stamped numbers and had an 848 head in good shape with rotators and properly drilled "steam" holes.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.