Regarding finding matching number parts, looks like it's more common in Lugers than in C96s. The reason is simple: there are many Luger parts still available. Just find a needed part is not a challenge, people want to find matching number parts, that's very normal when lots of parts are still available.
In C96s, we are looking for M1930s with banner stock. Stock is unnumbered, we can merry them together. When a gun comes with correct stock, that's more attractive than a naked gun. I just bought such an item (as an investment). They look pretty together. That stock may or may not sold with the gun back in 1930s, but they are comrades now.
http://www.gunbroker.com/item/660511509
In C96 domain, finding matching parts is like mission impossible. There are many parts coming from Chinese import pistols. Even if the number happen to match, its condition is usually too low to be mounted on a nice sample missing that particular part. So, this practice is relatively uncommon.