There is a picture of a Seitz bass with rounded lower corners towards the bottom of
this page. Again, probably a little late for what you are looking for. Of course, that page also show the famous "Bussetto" formerly belonging to Rainier Zepperitz.
As I've mentioned on that "other" bass site, I believe the rounded lower corners and f-hole designs on that instrument are derived from the viola d'amore family. Another thread here showed a bass by
Woodbury and Burditt that also shows some of these more exaggerated viola d'amore type features (for example, compare it to this
viola d'amore), but with standard f-holes. This bass makes me wonder if at least part of the inspiration for the rounded lower corners on many early American basses came from true viola d'amore style instruments, rather than simply Mittenwald basses with rounded lower corners.
Here is a modern reproduction of a violone made in Nuremberg in 1640 with strong viola d'amore features as another comparison to the Woodbury and Burditt instrument.
Incidentally, while looking up some stuff on Woodbury and Burditt (and it seems their basses may have been actually made by
William Conant) I found this interesting notice from the "
Fourth Exhibition of the Massachusetts Charitable Mechanic Association" in 1844, mentioning a musicial instrument competition that included instruments from Woodbury and Burditt, Prescott, Dearborn and others.
This notice from their first exhibition in 1837 mentions basses and cellos by Prescott, the Dearborns, J.B. Allen, and Henry Prentiss. It also mentions that, regarding the basses and cellos:
"They have been constructed upon such models of instruments of foreign manufacture, as the makers chanced to meet with; and, where these models have been departed from, the changes have all been made without reference to any other use of the instrument, than as mere accompaniment to the voices in common psalmody."