Such a bolt hold-down spring is neither unique nor novel for the 1920s. If the bolt handle of a cock-on -closing bolt action is inadvertently raised, the bolt will jump open and eject the loaded cartridge.
To prevent this, gunmakers sometimes added similar devices to such actions. Here is a photo of a very early cock on closing Mauser 97 transitional action, Mauser commercial serial number 17, used by Sauer & Sohn to build an early sporter, with such a spring added.

I know two such early S&S Mausers, both with such devices added on.
All commercial production Mannlicher-Schoenauers have a flt spring built into the right side of the receiver bridge, serving the same purpose.

BTW, this spring distinguishes commercial production M-S actions from the military production M1903 actions.