Tim, my "guess" is based on the information & images John posted above to include no captain bars, a 1959 casting date and the same casting number, 3733950.
John said; “ At any rate, if you find a 261 without captain bars, the casting code will tell you what it is -- either a block cast in 1954 or 1955, or a later block cast in the '54 mold. In the OP's case, if he's found a later 261 out of the '54 mold, then he's found a crate motor that was built as a replacement engine for the '54 and '55 first series trucks. What that further means is that it is a perfect upgrade engine for the AD trucks as it has the higher water pump position and fits in exactly without modification. So actually, for a pre '54 AD truck, the non-captain bar 261 is the one to have as it will require zero modifications to fit and work perfectly in your truck.”
I rechecked for a serial number and can't see one. The attached image shows the area on the pan rail between the starter and distributor (the bolt on the right hand side of the image is a distributor hold down bolt).
Thanks for your help Tim, is there another location I might find the engine number. Will spend time tomorrow when I unload the engine looking at details with a bit more care than today's hurried agenda allowed.
