Pohlman photos: the 2 XLRTT shots are dated Aug 28th 57, The 4 shots C/CH (2 C and 2 CH) are dated Sept 25th 57.
Earliest bit of info "outside the factory" I have is a 2 pages central spread on C/CH in the Feb 58 issue of "American Motorcyclist", repeated as well in the March 58 issue (... That's an "all States" magazine, so much for C = California only!!!). 2 small photos in the March 58 issue of the Enthusiast show us 2 off 58CH, one as it left the factory, no lights, the other with added aftermarket lights. Both have the "race flag and Vee" supposed 1959 logo on the gas tank and the race cut down front fender, just below the bottom stay as per above photos. I also have the recto-verso factory leaflet on them, but its not dated. There was also a full leaflet "all models" in 58 that features the C and CH.
They're all on my site:http://www.harleykrxlrtt.com/1958_xlc_xlch.htm
If you look carefully at the photo of the CH above, one can clearly see the proof that it's indeed an XLR engine: the "dynamo" blank plate is actually fixed to the threaded holes of the magneto adaptor plate, more or less in the vertical, not the dynamo normal holes which are at roughly a 60 deg. angle from these...
Another hint is the number of screws on the tin primary. The street XL had only 9 bosses in the case for these, the XLR stayed as the KR and have an extra boss hence 10 screws there...