Mike, welcome to the forum.
Not sure what a typical no marking base is.... is it black bakelite, or wood or aluminum?
Sounds like two GI's sitting around cleaning weapons and got parts mixed up or a put together from parts.
The numbers being close is good, but could be coincidence?
The sideplate looks like a erfurt (WW1) era sideplate.
Do you know of any story of how your dad got it or anything (see my signature for why
)
The 1938 is the year, the S/42 is Mauser and the land to land grooves is the 8,81 marking.
Ed