They could have other non-automotive uses as well, such as protecting threads on bolt ends, etc. etc. They are made of special heat-resistant EPDM rubber. Here's a list of the tubing sizes that they fit and the number of each that are included.
100PCS Carburetor Vacuum Rubber Cap Kit | Intake Line Plug Assortment
160pc Carburetor and Vacuum Rubber Cap Plug Assortment Kit Intake Vacuum Line
(100 PCS) CARBURETOR & VACUUM RUBBER CAP PLUG ASSORTMENT KIT INTAKE VACUUM LINES
CARBURETOR & VACUUM RUBBER CAP PLUG ASSORTMENT KIT INTAKE VACUUM LINES 100pcs
100 Pcs Carburetor & Vacuum Rubber Cap Plug Assortment Kit Intake Vacuum Lines
