Neither did my '66 Cessna 150 have a filter. Continental said dump the oil every 25 hours. If I had an oil filter the interval was extended to 50 hours. I'd rather change every 50 hours. That's close enough to 2,500 miles to seem reasonable.
W120 a 60 sae aero oil which I used to use in Pratt and Whitbey R2000 radials actually had a life of 100 hours, the oil filter on the R2000 was pretty good at stopping large rocks the occassional small child and about half the metal from a big end bearing failure.
So 50 hrs is a very conservative change interval, but Ill bet the Cessna didnt carry 20 us gals per engine.