The 400 and 300 series pens were based on models from 1937. There are reports that in 1937 a set of pens based on the 300 and 400 celluloid fountain pens was made for the owner in Sterling silver and gold but the precious metal pens never made it into regular production at that time.
The modern 300 and 400 series appeared in the 1970s and were available in Sterling silver, vermeil, enamel and enamel with vermeil.
That first Montegrappa turned out to be one of the 300 models. While the 400 series pens had vermeil furniture, the 300 series had Sterling silver clips, sections and cap bands. Initially the enameled 300s and 400s came with a masked gold plated steel nib and only the precious metal pens had the gold nib. However the enameled examples I've found have had the 18K masked nib.
And a Lined body with Charcoal cap Duetto
Here you can see some of the 300-400 series pen.
And the vermeil furniture on the 402 series.
The 300 & 400 lines continued right through until the early 90s when they were incorporated into the Reminiscence line but the new lines, the Memoria, Parola and Uno owe much to these early pens.
There are links to some of the catalogs of the period in the pdf section.