it's the lock on the storm window. It's a plastic piece it clicks in place.

Hello,

Thanks for reaching out! I believe you may need the 91-054 Nylon Slide Bolt Pair to replace your slide bolts. You'll need to remove one of the existing bolts and compare the dimensions to determine if this is the correct replacement.

These are used with the 91-003 Spring with Nylon Retainer.

Alternate image for 91-054