Poppet fits into Firestone models A and R and John Wood models RA and RC.
Worn poppets leak and should be replaced once the poppet cannot be properly reseated. Many different manufacturers have produced soda kegs and multiple designs have been available on the home brew market over the decades.
All poppets are not created equal and are not interchangeable. Trying to fit a mismatched poppet into the plug will typically result in damage to the improperly chosen poppet. Check for the manufacturer name and model # stamped either on the stainless steel side or the underside of the kegs rubber base to guide selection of the correct poppet valve.
Beyond identifying manufacturer name and model numbers, visually compare your worn out poppet details to the product image shown above to properly identify and choose the right replacement parts to fit your keg.