My Husky M38 is all matching, and yes, all of the numbered parts use the last three digits.
The stock usually has the entire serial number stamped in the barrel channel, same with hand guard, usually forward of the rear sight window. I have seen some replacement stocks and hand guards from other rifles where the new serial number is written on these parts in black pencil. Strange co-incidence that your replacement stock's butt plate serial matches the first three digits of receiver's serial number.
My belief is that when M96's were converted to M38's, the workers assembling the converted parts into rifles at Carl Gustaf Arsenal didn't bother with trying to match up the numbers of stocks and barrels to the receiver and all of the other parts. That would have been hellishly time consuming!
As for the Husky M38's, if they needed parts replaced, they simply used whatever was on hand no matter which factory made the part. The Swedish Gov't did not distinguish between the two makers. To them, M38's were M38's no matter who made them.