a screw exist just below mid hight of the primary twards the front - you fill it till the oil drips out with the bike straight up - puting in just short of a quart is also what one can do
All IronHeads primary oil either 1.5 pints == 24 fl oz, or until it drips out as mentioned by johnjzjz.
BTW, the 1980 and 1981 take 4 quarts of engine oil, not 3 like all the rest. The 79 tank looks like the 80-81 but is not the same.
Hmmmmmm........ I have an 81 XLH with a build date of 11/80 and I can't get more than 3 quarts into it. If I go for 4 it will certainly overflow. I wonder what's up with that?
I would guess that the factory was using up previous year parts. Post a pic of the right side tank area. The '79 tank has a different configuration for the filler spout. I might recognize it. I had a 1980 for 6 years, and have a friend who had a '79.