Got a 55 in the shop that has not ran since the mid 80s and it spins over but no oil to the rocker arms so far,think its a 56 265 and has a oil filter and staggered bolt holes so it could be older then 59. It has a distributor with a window to adjust the points that I though came out around 57? so I thought I read that 55 and possibly 56s took a different distributor that is needed to get oil to the upper engine.
The rear journal of cam is the portion that is different. It has a notch to receive oil then distribute it to lifter galleys. Rear cam bearing is where transfer happens however it's not constant so that's why it may take so long to get oil up to lifters or current cam isn't notched and or rear bearing? Distributor isn't an issue in this case.
Your right 55 and 56 265 had a special distributor with a flat spot on the lower housing for oiling. The cap has snap clips to hold cap on and no window. In 57 all that changed.
55-56 distributors had pressure feed oiling. This was controlled by the rear cam journal being pulse feed. The flat spot on the side of the distibutor was purely for oil drain-back. The flat spot must be "clocked" so the R/H lifter gallery doesn't lose pressure. The 57 and later distributor is splash feed, and doesn't require the same drain-back. you can run a 57 and later distributor in an early 55-56 engine as-is, But if you use a 55-56 distributor in a later engine [with constant feed to the lifter galleries] it will pump oil up into the distributor cap. If a 55-56 engine has the "rear cam journal" oiling mod ,it will also need a later distributor.
Flat is not for oiling. It needs to be clocked so flat is opposite side of lifter galley. The closed grove is where oil transfer happens as shown here.
If I remember correct, guys liked the early iron distributors better than the aluminum ones for a better advance curve. Been a while since I played with chevys