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These rather common screw locks were (and sometimes still are) used by railroads for low-security applications. But I haven't been able to find out very much about their history.

I haven't found any patents (tried searching for "screw lock", RACO, etc.).

I haven't found out much about RACO, possibly an abbreviation for Railroad Accessory COmpany (except an implication they were once known as SACO).

Safetran still markets these, see http://www.safetran.com/product/Sign...IG%20K-6-1.pdf

There is a bit of history that I think involves WRRS here but they no longer make screw locks: About Western-Cullen-Hayes

Can anybody fill in historical background and/or patents?

Jim