I'd order the newer, if you think you have a 55-57
the reason for them showing different parts for the same year is options or model differences, like car/truck or 'heavy duty', and the fact that some 235's had solids, some hydraulics - the oil pump difference is likely to be the later one is higher pressure - your example there doesn't actually say that they're "1st" and "2nd series", and it's likely they actually mean 'pre' and 'post' 55 - I don't know of any real change in 55, but higher oil pressure could be one, and there may have been a different Rochester carb number standard - the 1st/2nd designations had to do with body differences, the 55 1st series truck was effectively a 54
the 235 that was used in the TF series trucks, 55+, continued the standard version that started in 54 [sometimes called "high pressure 235"], but there were 2 variations, the "thriftmaster" [109hp] and the "loadmaster" [105hp] - there were a number of different details every year, so you need to identify the year if you start doing major work, but you've said the number identifies as April 1, 1956, so there's the answer, and as I said above, if the engine was built in 1956, it's not from a 55 sedan delivery, tho the same casting number may have been used then - what would id it as from a delivery would be the emgine number [pad by distributor], not the casting number
Bill
