It appears that XL rockers are made in pairs like the pan & shovel (but no knuck): rear exhaust is also front intake, rear intake is also front exhaust.
However, I have more varieties than this. Some have an oil spit hole, some have the hole welded shut, some don't any hole. Some castings (forgings?) are clearly different, the number size varies, some don't have part numbers, etc.
I also have strange part numbers - what appears to be a set of 4, all equally dirty etc. as if from the same engine - with 2 each of 17389-57, and 2 each of 17392-57.
Since there is no other rocker change in 1957, what happened to 17390-57 and 17391-57?
I have another partial set with no part numbers at all, but cast-in numbers as follows: the ones marked 20 or 25 are the same shape but one has an oil hole and one is welded shut. Clearly, the welding was done after the number was cast, so why are they different?
I suspect that at one time there were 4 different rockers (not 2), and the factory decided to save money by just using 2 when stock was depleted to save money - and to reduce errors when a shop was working on this since the alignment will be wrong if you accidentally substitute a front exhaust for a rear intake etc.
Does anyone have a Shop Dope from the period 1957-64 that covers this?
How about an original parts book from 1957 with the part # and use?