After relooking at your pictures and diving through all the casting number resource links I have saved over the years, I agree that it does in fact look to be "3733813". My apologies as there is also the very similar casting number 3738813 I mentioned, which is where my brain thought it need to go.
With that in mind, looking at the following casting number resource lists, all of them indicate that this casting number "3733813" is for a "1958" 261, which corresponds with the block date casting code.
Jim Carter 6 Cylinder Casting Number DataGreg Wapling's 6 Cylinder Casting Number DataDave Dawn's US and Canadian 6 Cylinder Casting Number DataBig Dog Garage 6 Cylinder Casting Number DataI'm interested to know what source you found that indicated it as a "1956" block???
I did try and confirm through various Chevy Master Parts Books, for both of these block casting numbers, and couldn't actually find it listed in 1957 or later 1959 books. I don't currently have a 1958 version to check, but plan on try to find one eventually.
Bottom line is either way, you have a 261 and that's a great thing IMHO.
