Maybe ISMACS got it wrong? That serial # listed on their site shows the result khogue mentioned above: 15K-Jan-June 1905. No clue about the stitch length lever stop. I know my '26 15K has the rotary stop plate like the one shown in the OP's pic. About the irons... doesn't mean this head hasn't been fitted into a different base. I'm going with the ISMACS listing, which has no "JC" prefix in their database- only "J."