From all the information I have I still can't figure out what was factory and what not. As far as I can tell there have been three versions for the XLs to the late 60s -- one for the XLHs (casting nuber ends 56), one for the XLCHs (ends 58) and one for the XLRs (ends 57R).
The K-models seem to have three castings as well -- one for the early K and KRs (casting number ends 52), one for later KRs (I guess 56 and later) and one for 56 KHs (ends 56) -- plus the 57R for later KRs.
So far so good -- confusing is, that all the wheelbases are that different, that there must be a difference in rake plus the fact, that the earlier castings seem to have less metall between lower end of the steering tube and lower tubes. I added some photos to show what I mean (Patrick, I hope you don't mind using some pictures from your site ...). Still you can find later KRs or XLRs with the "smaller" Head casting, that looks like the earlier Ks ....
Now, another thing is that the non racing frames from 56 on are "gooseneck" style (what is actually the difference between the KH/XLH and XLCH frame neck casting -- as they have different endings?). If you look at the 58 XLH frame I have, you can see that there is some kind of "optical extension" of the lower tubes from the front tank mount to the top of the steering head.
If you look at the 56 KH or propably KR frame I have, you can not find it although the neck cating end with -56 (The bad thing about this is, that the former owner grinded the number away, thinking it would be the VIN -- so I can not compare it to the XLH frame).
For me it seems that there must have been at least another version of frames (my frame is chromoly, but not thinwall, has K-model cast motor mount but clearance for a XLH engine, intermediate tube extensions on the front lower tubes, dated A6).
Then you can find racing frames with "steering head job done" mostly lowered or deraked, but also versions with bigger rake for "stability at speed".
If you look at Patricks KHRM photo from the KR frame post you can clearly see the intermediate tube extensions and it looks like a gooseneck casting (from the height of the upper fork tree) ... you can find the same frame in Allan Girdlers "Harly Racers" on page 50. It is a picture from Fumio Itoh on a KHRTT -- you can clearly see the gooseneck and it seems to have intermediate extensions.
Still not clear what steering rake was used, was it common to change the steering head on racers or even factory standart? Did they grind the lower area of the casting? How many castings have been used?
I now this might be a look and very specific posting, but I think this is the best place to ask ....