8 3/4 rear end 489 date code strange

[Ifixmycarmyself]

I'm planning to buy a 8 3/4 489 but the date code looks strange... 275. Can't be day 27 in either 1965 or 1975. the didn't make them before 69 or after 74. so what is it? Was hoping someone here could advise since asked price is 

[6bblgt]

275th day of some unknown year - what is the date on the ring gear?

October 1st of leap year ('72) or October 2nd of a non-leap year

[Ifixmycarmyself]

275th day of some unknown year - what is the date on the ring gear?

October 1st of leap year ('72) or October 2nd of a non-leap year

Thank you for your reply!
Axle is restored and seller is not to keen on pulling it apart to check ring gear. On a better picture we can se the digit 1. Could this be a split date code like we have on intake manifolds and such. Were the digit 1 could be the year 1971 and 275th day. I have some code books, also the David Weis books, but they do not cover such coding but the 275 without year doesn't make sense.
What to believe??