PWC9 Upper

I see mac-11 uppers for sale at:

1. Aves
2. Parts Dispensed
3. JC Arms
4. MasterPiece Arms

Since MPA only has the side charging upper immediately available, can I purchase a side charging bolt from another vendor ? I'm guessing MAC-11 is a specification as well, so parts should be interchangeable.If not, whats the opinion of the complete uppers from Aves ?