Were they popular for these rifles, with the Monte Carlo style stock during that time period? The ones that I have seen, were very well fitted?

What rifle? I have one on a Mod 88 Winchester that was finished by a gunsmith. Very nice wood.

I’ve seen a few 99s with Fajen stocks but not many. Companies like Reinhardt Fajen and E.C. Bishop did a lot of trade in aftermarket stocks in the 50s and 60s to guys who were buying surplus Springfields and making them into Sporters. However, they also cataloged stocks for other guns such as the 99. But those guns usually didn’t need complete makeovers like the milsurps as they started life as sporting rifles. I don’t think the stock companies did much fitting themselves as their products were mostly aimed at the DIY market but it’s no stretch of the imagination to figure a few were fit up by competent smiths and woodworkers.