IF the shape is named after the maker[/qoute]
Well, that is one of the main points I am trying to make besides the spelling. I don't see how the name of the shape is named after the maker in question since there is no proof that G.M.Busetto ever made a Bass. The one Bass mentioned attributed to him is highly questionable.
In that case, origin and spelling go hand and hand. If we knew why the shape came about or who 'actually' did it first, then we might have an idea. It is possible though that given the period of the maker Giovanni Maria del Bussetto, he may have made a Viol d'Gamba with a defined lower rounded corner that we now refer to as Bussetto/Busetto/Busseto style.
Many of the old makers including d'Salo, Maggini, Amati and even Strad to name a few of the more famous ones, made Viol-family instruments besides Violin family instruments.
The earliest use I know about of the "lower rounded corner" as referred to this name by Raymond Elgar in his book 'Introduction to the Double Bass', he shows a Bass by Matthias Klotz on pg.48. The dates given for this maker, the oldest I know about with these 'illusive' corners is 1656-1743. The Bass pictured in the book is considered to be a "the True full-sized" Bass measuring; Belly - 47 5/8", UB - 21 7/8", MB - 15 3/4", LB - 30", Ribs - 9 3/4"@LB to 9 1/4"@UB to 6 1/4"@NK, SL - 43 3/4", Total length - 79".
This Klotz is slightly bigger than the biggest Bass I own which is huge in itself. I can just imagine carting this thing around town to gigs..
The Top & Back of the Klotz are both only 2-pieces of wood, matched from a single extra wide billet. Only the Top is purfled. My Bass is the same in this respect but considered to be English with Germanic and French influences. My large Bass has soft almost Guitar like lower corners as if the Bussetto 'bubble' is just missing. An obvious 'Klotz-like' imitation but Englishized in its own way.
Of the lower rounded corners we have come know as 'the Bussetto' (pick your spelling) all of the early German makers that made these did them in 4/4 dimensions. The later Neuner-Hornsteiner Basses have often been seen with smaller dimensions but this is almost 200 years after the model came into play from what I have found.