Dropout/axle spacers have been discussed several times here:

Adjustment screws for rear drop outs
Can someone tell me what this thingy in my dropout is for?
Rear Axle Position On Older Bikes
Rear Dropout Adjuster- Why & How
Two quick questions about dropouts and adjusting wheel position

If you currently have one then you're all set. Schwinn and many others used only one on the left side of bikes having dropouts with integral hangers.

The one pictured in the OP is similar to the Huret version, but from a different manufacturer. A Huret ref. 849 "alignment stop" recently sold on eBay for $13: Vintage Rear Drop Out Spacer Axle Used | eBay