got a pair of these webers. what size motor do they suit, are they a good setup , easy to tune, fuel economy etc?

Up to say a 2L with 36 vents

Are they good?? They are the pre-design to IDF's.
"Apparently" they slosh around a lot more than IDF's
Berg did a modified version in the 60's that were the best thing since 6.9:1 CR and semi hemis.

hard to get parts for now.

Berg loved them! do you have 3 stud or 4 stud bases. used by ferrari and lancia for their hipo models. more bang per cc than idf's but have a weak float control system. do not like high fuel line pressures or inclines (no baja's)as they will flood. for old skool horsepower hard to beat.:bounce

Does anyone know when exactly the DCNF became commercially available? IDF?

I run a set of 40's on my 2 litre and they work great. I've only had to fine tune them once in over 12 months.