The type numbers differentiated carbs manufactured for specific applications. For example, 27 was for a 1600 Giulia GTV/Duetto while 32 was for a 1750 and 18 was for a Lotus Twincam. Most of this had to do with the calibrated components provided by Weber (including all jets, venturis, but also the rods that determine the pump stroke) and also altered the number and position of the progression holes, which is a major factor when adapting carbs to a specific application.

Not all variations are easily transferred to other applications; for example the types 23 and 26 were a matched pair intended for the Renault 8 Gordini on which the installation was very tight. The throttle shaft ends are a very compact arrangement that could not be used on an Alfa. Similarly the types 34 and 35 were another matched pair designed for the Sunbeam Rapier H120; they are missing a breather hole in the float chamber. They are said to adapt very poorly to other applications.

Around 1980 Weber introduced the type 151 40DCOE (and type 152 45DCOE) with plastic floats; these are regarded as a great evolution and are still available today. Production was transferred to Spain in the early 90s, which does not alter their desirability. They had initial problems but after 30 years I would believe the bugs have been worked out. These are generic aftermarket carbs, not designed for a specific application.

Alfa made cars with twin Weber carbs until the early 90s. At some point they switched to the DCOM, a variation on the DCOE theme that included a diaphragm acceleration pump and other emission features. These are not regarded as performance carbs and are better left to their intended applications.

There have been numerous copies over the years; we all know the slightly different Dellorto DHLA and Solex PHH as well as the Japanese Mikuni Solex. At some point Shankle was offering Japanese SK clones of the DCOE, which used most if not all of the Weber calibrated parts. These are still made in Japan under the OER name.

Empi markets Chinese copies of various Weber carburators. I haven't seen Empi DCOE clones, but some are marketed as FAJS and GOWE. From what I understand these are best left alone.

 

Good to know that specific carbs were made for specific engines.

The resourceful Italian engineers and designers worked with out without the modern simulators and diagnostic equipment that is available today.
Fluid dynamics and fuel atomization research for different engine displacements and varying engine speeds.
Not to mention that they were often working with lower, and varying, quality fuel as well.
Very precise casting and drilling and the use of various types of materials.
Aluminum, brass and steel, gaskets etc.
A lot of it was trial and error.
How many clones and "nearly the same" carburetors are there out on the market now?
Imitation is the sincerest form of flattery.
Impressive!

 

It is a pity that there is no written history (at least none in English) of Weber carburetors. They are talked about in some books about GP racing in the 50's when they were very expensive, custom built devices. At some point they went into volume production and were OE on many sports cars. Most of the discussion on here is about DCOE's but most of their models were down drafts
There have been threads on here about the differences between "classic" and "emissions" Webers.