Paul - A question with a little different thought track on your engine. Is your truck/engine by chance a Canadian Chevy???
If it's a Canadian engine, the block casting number is most likely just above the oil pan rail on the passenger side, in front of the fuel pump location. Below is a snapshot from another Bolters thread on where to find casting codes on some Canadian engines.
Take another look at yours and let us know what you find. If it is a Canadian engine, that could account for the different stamped serial number sequence since yours doesn't match the codes in the US made trucks data file.
PS - Canadian 235's and some 261's also used the "848" heads o there are some similarities to US Chevy's.
