Thirlwell,
As stated above by Fab51, the "low-pressure" 235 engine was available on large trucks from 1941/1942 through 1953. The high-pressure 235 truck engine (first used on the 1953 car with Powerglide automatic transmission) was used on trucks from 1954-1962.
I will check my shop manuals tonight to see what size 1951 trucks had the 235 as an option.
Your logic about (I don't think anyone would have upgraded by putting in an old splash-oil engine once the full-pressure models came out) agrees with my observations and experiences, unless a free (or almost free) engine was available.
If someone had an old truck with a blown original engine, and needed any replacement engine to get an outdated truck back on the road (at a low cost or for free), and a low-pressure 235 was available at a good price, the low-pressure engine would be put in without a thought about looking/paying for a high-pressure 235/261 (or considering to use a 216, which could be had a low cost or for free).
By the early 70's, high pressure 235s/261s were readily available from salvage yards at reasonable prices ($50-$100 - prices that I paid at that time). Nonetheless, I also put cheaper/free low pressure engines into fiends trucks - the price was right and the labor was cheap (a meal and a siz pack).
As khardy stated, post the engine serial number (stamped into the ledge behind the distributor) and/or the block casting number (I think it is behind the upper right side of the starter) and it might be a little easier to guess the year/size of the truck from which the 235 came.
Tim
