Gil; I the Ideal #27 handbook two roll crimpers are listed which have the reversible pins, the diamond & the Star. The Star looks identical to your crimper except on the one pictured in the manual the star down on the frame appears to be either painted on or a decal, rather than cast in as on yours. I suspect yours is an earlier version. The reason stated for the separate shell header sliding on the two rails was that it would handle any length shell in a straight line fashion without a tendency to bend the shell.
It was also stated that until the Coming of repeating shotguns the square crimp was universally preferred as it gave more resistant to opening, giving better powder combustion. The rounded crimp became a necessity for repeaters to prevent hanging on the chamber in feeding. The pins on mine & appears the same on those pictured in this #27 manual are secured by set screws entering the face of the head. y crimper closely resembles the Diamond crimper pictured & does not have the separate header nor the rails.
This #27 was published by Lyman & is I believe the first one they had published after purchasing the old Ideal Co. The one I have is a reprint & does not include a date, only the number, but as I recall this took place in the mid-1920s
I have an original #37 Lyman/IOdeal manual & it still lists the Star crimper but the base is now different, the New crimp head does not use pins & it now has a belt drive designed to speed up the crimp head. The Straight line loading set is also listed. I do know that both continued on the market through most of the 1950s but not sure just how long.
