David, I checked my closest is
Pre Model 17, Satin Blue, 6" One line address, knurled ejector rod end (no tip)
Serial K 18529 Shipped Feb 1948
It has no barrel cut for a rod end so must be a shadow.
Ralph, my closest two to yours are:
Pre Model 18 , Bright Blue, 4" (5 screw ) Serial K 246498 Shipped May 1956
Pre Model 17, Bright Blue, 6" (5 screw , Wide rib Serial K 259018 Shipped June 1956
Since these two flank your guns Serial but have later ship dates I have no explanation or guess as to why yours was satin blue except perhaps it was at the very end of the satin blue K22 era so squeaked by , being that I have not observed a satin blue 5 screw wide rib K22 to date and that yours is no longer satin I'd say it's a bit sad if it lettered that way since it would be a rare variation IMO.
On a side note the five screw wide rib variation has a very short production run when compared to the 6" four screw which was only offered in the wider rib barrel style, the pre model marked four screw is also a very short run as Model marking came out shortly thereafter,
Another thing to consider and compare is that post 1955 holiday shutdown and when the factory reopened in Jan 1956 the then new first four screw frames on the new Combat Magnum were being assembled and shipped in bright blue alongside five screw varients of other models.