Thank you Chris and Olaf for pointing me in a new direction with this "Alfa", which is looking more and more like a Maserati Tipo 4CS 1100 , possibly chassis #1123 before it was rebodied in 1977 (and then again in the 1990s). I always thought it was a special bodied Alfa Romeo, since that is what I wrote on the back of the snapshot. But that was 34 years ago and could be a lapse of memory. The photo is too indistinct to make out the badge on the nose, but under a loupe it looks like it could be either an Alfa or a Maserati Trident logo. See this link for Detail description of #1123 on www.barchetta.cc
There were only thirteen of the Tipo 4CS cars made in two seat configuration, with 1100 and 1500 engines (Count Giovanni "Johnny" Lurani routinely swapped engines in one of his in order run in different classes). Hence I was able to work backwards through a process of elimination to zero in on #1123 as the most likely candidate. Another possibility would be a re-bodied Tipo 4CM (monoposto = single seat) but so far they all seem to be accounted for.
There is a striking similarity between the unknown "Kruse" car and the second car above (#1124, with closed fenders), which is the well-known ex- Briggs Cunningham / Rosso Bianco Collection car which sold at Bonham's Monaco auction in May 2006 for €643,000. It was originally made for the 1935 Mille Miglia; see the following link with
Details in Bonham's program notes for #1124
In addition to the usual printed sources (principally Orsini & Zagari's book) there is a lot of fascinating detail in a thread on 4CS and 4CM on www.Ultimatecarpage.com forums
where I have also posted some inquiries, which may lead to some more definitive answers. Meanwhile there is a chance that the Kruse people may find some more materials in their archive which will shed some light on things.
The only thing better than a good mystery, is seeing it solved. And I think we're getting closer.