Greetings:  I new collector was asking when the factory began punching holes in the flywheel housing for direct electric start; His machine has pony start, but wondering just in case he decides to go DE for no particular reason. His machine is 81XX serial I believe. Thank you. JM

5U14695 had the first hole for the starter.

Thank you Rome: I really was not sure if there was a serial break when they put the hole in, or just at random. JM

I am headed for the ranch right now. Will be poking around my brothers 5U to see if it has the starter hole. I did not know that any D2s had them.

Direct electric starting for the diesel engine was an option for most all of the D2 series, starting with the early J models, The drilled bell housing was part of the electric starter attachment package up through D2 5U13236 and 4U 6372, after that the drilled bell housing was standard.
You may find drilled housings on engines with pony motors prior to 5U13237 etc, as I thik some got changed back to pony start, because the early electric start preheat systems were not as reliable as the later glow plug systems, and as the tractors aged the pony start was a better option.