Back to the "nasal extension" - there were actually three different versions of that, all pictured in Günter Engelen's excellent "Mercedes-Benz Renn- und Sportwagen" book of 2011. One had a very modern look to it, and I'm sure I have ssen something similar on a 70s or 80s sports car, maybe a Rondeau, or a Matra-Simca? Unfortunately, I'm crap with sports cars.
MB tested at Monza Aug 22/23 (Aug 22, 24 & 25 according to a different source) with four cars and six drivers (Fangio, Moss, Kling, Taruffi, Simon and Fitch, plus Uhlenhaut?), but the only picture I've seen in the period press was from a monoposto. According to the works records, chassis #2, #6, #14 and #15 (with engine #11, #15, #12 and #13, respectively) were present, although no mileage was recorded - a bit unusual, but not without precedent. Nominally, only chassis #2 ran in streamlined form, but we know bodywork could be exchanged easily. There was another test on Sep 7, Wednesday before the GP, and according to the works records only cars #2 (engine #20) and #9 (#16) took part, both with streamlined bodies, and then for the actual GP chassis #2 (engine #25), #6 (#23), #9 (#16), #10 (#23), #12 (#21), #13 (#25), #14 (#17) and #15 (#19), all streamliners except for #6 and the last two. To make matters even more confusing, the three oldest cars had the long wheel base (2.35 m), and the newest one the short (2.15 m), with the other cars in between at 2.21 m. In the end, none of the "middle" wheel base cars ran in the actual race, and Fangio won with long wb/streamliner #2/25 from Taruffi in short wheel base/monoposto #15/19, with Moss (lwb/str #9/16) and Kling (lwb/mon #6/23) retiring.
In "Auto Motor Sport" 20/55 there are a couple of pictures showing the "nasal treatment", without making clear whether they were from the Wednsday test or the actual GP practice, however. One is particularly interesting, showing a monoposto body with the same #20 lying beside the streamlined version of the car (with nasal treatment), indicating that a switch of bodywork was a matter of mere minutes. #20 was Kling's number in the race, but if the picture was from the test on Wednesday that doesn't necessarily mean much. According to Engelen, the airbrake was only tested in August.