Can you identify this Darby Handcuff?

This Darby cuff has no name, but it does have a "441" and a very faint "HARD" stamped on one bow. The end caps are pinned. The key is one I made to fit. The lock post is 1/4 inch diameter by 12 Threads per inch. I threaded a drill rod with the correct TPI and made that into a tap. I then turned the key body, cut the internal thread and welded on the handle. I can clean up the welds to improve the appearance.

Can anyone identify a maker and general date for this cuff?

Could be made by what were known as "Saturday Nighters", that is, employees making their own cuffs after hours. Looks like any Hiatt, Froggatt etc.
Paul