The Appalachain Autoharp was first built on the "A" Model frame around 1964-1966. Sharp corners. Basically an unchanged frame for more than 60 years at that point. The more curvacious "B" mdel folloed a few years later. I have one I bought in 1969. Powerful box for an Autoharp.

The dates are all in Becky Blackleys fine book.

Mooh the one your Mom had sound more like a Zither than an Auto Harp.
Probably had 3 or 4 courses of 4 strings that were tuned as achord. Then an Octave and a half of chromatically tuned strings.

Don