I tried to do a quick search and did not see anything I had not already found....just in case some one has some new wisdom to share...
Motor is in a late 60's early 70's garden tractor
Tecumseh HH80 110089C 8312
From what I am reading, am guessing 68 and 312 days into the year???
First digit is year, next 3 is day of year - from the 4 digit serial number. So it was built the 312th day of 1968. Likely the tractor would be a 1969 model. This way of dating only works if you know the decade they were built in, as the cycle begins every decade on the year digit.
This also fits with the years you have found on build dates for the V4-8.....first year of production in 1969.