The Wm. C. Stahl was a publishing firm out of Milwaukee which, in addition to selling music, sold instruments from other makers (all while advertising themselves as the maker).   It is commonly agreed that Larson Brothers in Chicago were the principle makers of Stahl Mandolins.

Stahl exhibited its first mando-bass in the 1912 Guild Convention and began advertising shortly after.  

However it is unclear which mando-bass that actually was.  Of the known Stahl mando-basses, there are two distinct styles.

The first is a highly ornamented bass which matches the advertising in virtually every respect.  From a Bernuzios Uptown listing:

“This example was sold by Wm. C. Stahl, who was a Milwaukee-based music publisher and retailer, and likely was built by the Larson Brothers. Features an oval shaped body with a 19-3/4″ lower bout, the top is spruce and is double-bound in ivoroid and marquetry as it the 3.5″ diameter sound hole. The back & sides are mahogany and are also bound. 39.5″ scale bound ivoroid neck with original bar frets, ebony fretboard with pearl inlay; pegbox has a crude scroll and geared plate tuners. Notable features include a unique a diagonal bracing pattern on back and top, extendable side end pin on treble-side corner, and original four string tailpiece. “

The second example shows a completely different body style, an thoroughly unornamented teardrop style which retains the crude scroll headstock. Label inside clearly attributes to Stahl. Mahogany back and sides, and likely spruce top.  Has both side and end-pins and large floating tailpiece.  Most interestingly it has a longer scale-length at 42 1/4”. 

Given that it so drastically differs from the “marketed” one above, it debatable whether it is in fact a prototype bass or whether Stahl followed the same pattern as Gibson and Vega and increased the scale length to account for concerns regarding tone and playability.   

In the following image from the San Diego Singing Strings this model (or this very) Stahl bass with a different tailpiece appears to be in the front row. However it remains unconfirmed.