Download the parts diagram from here. Next time you call, you can have a list of part numbers. Saves both parties a ton of confusion.https://www.crosman.com/american-classic-pistol-22
If you're looking for the little 4-48 screw that holds the front part of the steel breech to the pump tube it's 1322B027.The set screw is probably the one that secures the barrel to the breech.If you call them back and explain they usually will ship the right part for free.
Or you could order this one. And peruse their other offerings at the same time. Very reasonable prices.https://onlinestore.magnumairpower.com/products/custom-breech-screw