There is no mention of exchanging a new Sealed Unit Assembly for the old one. Instead, it states that the gun would be returned to the factory and the customer would be given credit for those parts that could still be used when repairing the Sealed Unit Assembly. See page 9 of the model A manual.
I really think that the entire reason for this assembly being removable is it's the only practical way of getting maintenance access to the check valve seat. I think designing in a ball bearing check valve is just short of crazy time, to start with. What's the point?
There's nothing cheap and throw away about the Sealed Unit Assembly.
Be sure to check out the complex valve cage assembly. Which tends to get torn up when disassembling the Sealed Unit Assembly.