I cannot decode engine serial numbers (some jerk tossed out my book) but if the rockerbox is held on with _4_ scres , it's a 235 or 261 (lucky you) .
The engine serial number is _stamped_ on a smooth pad just to the rear of the distributor .
Two acorn nuts on the top of the rocker box usually means it's the original 216 " Dipper 6 " eninge....
Find a picture of someone's engine and post it up , we'll tell you wich it is to compare to yours .