Early "Multiscale" Fretted Instruments:
The Orpharion, Cittern and Bandora (Bandora is shown illustrated below) were all built with Multiscale frets (or "sloped frets")
This technique was done long before (invented in 1562) it was ever seen on any guitar or bass; even before these instruments were invented.
The inventor was most likely an English Luthier named John Rose
The Bandora (seen below) was built in the year 1619 and featured a "Multi Scale" length of: 745 mm x 730 mm. It is a large (metal) stringed instrument characterized by its elaborate body shape, sloping bridge and frets. Although it's generally regarded as a member of the cittern family it also had some significant construction features in common with the lute [an early relative of the modern guitar]. Today the bandora is mainly associated late 16th/early 17th century England, but it may have been just as common or even more common in Germany. In fact one of the names it's known under - cetera tedesca - means "German cittern."
See some very beautiful reproductions of these instruments (with multi scale frets).