Shelf for a typewriter. Manual typewriters are heavy, the springs help move the shelf up and down. Once you move your shelf to the top, turn that thumb latch to lock it in place.

Some examples:

https://instappraisal.com/appraisal/antique-secretary-desk-slide-out-typewriter-shelf
https://www.myantiquefurniturecollection.com/i-have-a-vintage-myrtle-desk-co.with-pop-up-typewriter-shelf-awesome-two-…-53793.html
https://shop.blackdogsalvage.com/tanker-desk-with-build-in-typewriter/
https://www.ebay.com/itm/274785544688