My recollection is that, aside from the "proper" customs in Mach's list, there were a few years when many of the big dealers were offering "custom" jobs that were basically the stock action stuck in a CS or similar stock.
For example, I haven't taken my Gold Sierra apart (it works fine), but I have no idea whether Northern Arms did anything much to the insides. I sort of think they must have, as they would have to have taken it apart to plate the end block and underlever.