Over the last two decades plus I have collected information that I could find on down draft E. Weber carburetors between the 1950s to late 1960s. There were a lot designs and suffixes. I bought some factory manuals on theory, design, and practice. I did not make notes on every design version but I did collect a little information that more or less looks like evolution outline that culminated in the 48 IDA and 48 IDA1 two choke carburetors that became so famous.
I cannot guarantee that my lineup is correct chronologically between the 35 IDM 2C and 46 IDM 2C families. It is very likely that multiple versions were somewhere in the design to distribution phases simultaneously. So what little this is worth:
35 IDM two choke, sand castings, down draft – Maserati 250F and 350S engines.
38 IDM two choke, sand castings, down draft – Maserati “Birdcage” race cars. Mr. Shelby raced one of these cars.
40 IDF (not IDM) two choke, sand castings, down draft – for small displacement Italian engines.
45 IDM two choke, sand castings, down draft – used in the Maserati 450S (and perhaps other cars) circa 1956-58. (Carroll Shelby raced, as driver, one of these cars.)
45 IDM? two choke, sand castings, down draft – used on the Maserati 5000 GT street car for 1959 model year.
46 IDM two choke (new design, significantly different in many ways than the previous 45 IDM two choke model), sand castings, down draft. Two assemblies were used in Porsches that I have found reference for: 46 IDM1 (Porsche 1500 RSK) and 46 IDM2 (Porsche CARRERA) assemblies.
48 IDM two choke, sand castings, down draft - developed (originally created by reworking existing 46 IDM assemblies) for a Maserati F1 effort with very few sets made - circa 1960-62 .
58 DSF two choke, sand cast. Ford Motor Company developed an aluminum 255 c.i.d. push rod operated over head valve “INDY” engine during 1962 and into 1963 with four (4) each 58 DSF E. Weber two choke down draft carburetors mounted in a row down the middle of the valley between cylinder heads.
48 IDM1 or 48 IDM5 two choke, sand castings (Note: IDM1 and IDM5 assemblies are based on different main body castings), down draft - appears on anything Ford Motor Company / Carroll Shelby Enterprises / Shelby American, Inc. had anything to do with in V8 engines using down draft systems. Used until circa late March / early April 1964 for Shelby team cars. I have not seen any pictures but for there to be 48 IDM1 and IDM5 assemblies I have to wonder what suffix 2, 3, and 4 assemblies were? The 48 IDM family of carburetors found use in multiple brands of American V8 powered sports and sports racing cars before the die cast IDA 2C family came out. Some racers kept using them at least for a while after 48 IDA/IDA1s came out. A 1965 MUSTANG GT350 was raced in Canada with some version of 48 IDM carburetor before being converted to IDAs.
40 IDA 3C - (three choke, i.e. three venturi or 3V in America),
die castings, developed for Porsche (now three 40 mm throttle bores instead of two 46 mm ones).
48 IDA 2C (two choke),
die castings – The production ready Weber engineering drawings for the 48 IDA model were dated September 7, 1963. 48 IDA family models were considerably lighter weight than the 48 IDM family models and were assembled with 42 mm chokes. The first production units did not appear at Shelby American until very late in March 1964 or very early April 1964 the best I have been able to determine. Vague E. Weber published texts could make one believe the 48 IDA model was created and initially developed for what would become known as the Ford GT40 MKIs.
48 IDA1 - appears at Shelby American in the same time frame as the 48 IDA. 48 IDA1 models were manufactured with 42 mm chokes. The difference between models was in throttle arms supplied and that did not matter to Shelby American (or Ford Advanced Vehicle in the GT40) as they replaced them with custom new Shelby American (or Ford Advanced Vehicle) designs anyway.
46 IDA2 and 3, two choke - Edorado Weber manuals indicate they were for a Porsche application. The only 46 mm versions that I have come across in an American "factory" context were in a four carburetor small displacement aluminum V8 engine system that came out of a GM engine development operation. A single carburetor has been found in use in a hot rod. Visually they look just like 48 mm models.
48 IDA4 for Dean Moon. 48 IDA4 models were manufactured with 37 mm chokes. Dean Moon's Moon and McKay companies sold no telling how many systems for Chevrolet or other GM engine powered applications. (Personal correspondence from Dean Moon to a potential customer post marked June 28, 1966, includes an undated “WEBER THE KING OF CARBURETION” news letter advertising 48IDA1 carburetors for custom induction systems for a variety of American V8 applications, includes an undated copy of a typed spare parts list for “48 IDA – IDA 1 - - 46 IDA 2 – 46 IDA 3 48 IDA 4”. This is the earliest documentation that I have found to date on the “48 IDA 4” model. Mr. Moon indicated that IDA4s were for applications where IDA1s were too large. The next reference found was in a 1967 catalog. February 1967, GEON, WOODBURY NEW YORK, "48IDA4 COBRA (SHELBY)", no pricing available, under price is “P.O.A.”, printed in U.S.A. This is the earliest published catalog mention and only printed information during the 1960s related to the “48 IDA4” that I have located to date. This is a Geon publication and not one from Weber, Moon, Ford, or Shelby.) (Did you notice? Geon advertised assemblies with 37 mm chokes as replacements for models with 42 mm chokes in reference to “COBRA (SHELBY)”.) The 48 IDA4s were very popular in marine applications for a long time.