There's more than one question here - I'll start with the simple answer first.
(1) The markings on older Italian made accordions were applied 99% of the time by the US distributors that ordered
them. In the 1930's a law was passed that containers or products entering the US be marked with the country of origin.
Accordions were normally shipped in containers and the accordions they contained were not marked with origin.
In order to be sold distributors had to apply their country of origin by a tag or (in most cases) were scratched in their
finish.
As for serial #'s, distributors in most all cases applied their own. These #'s were applied before accordions were shipped
to dealers. These #s were (and still are) a distributors code that contained year of make, year marketed and info that
only the distributor could decipher. I've many times inquired for info on old distributor files and in all cases the answer
has been "Their existence is unknown or no longer available".
(2) The Stroller you have is a late 40's early 50's . The earlier Strollers had the same body & grill but with waterfall keys.
As far as the keyboard size - Italian accordions were made in metric sizes and the US were marketed with a size rounded
off in inches. Student models 14" 15" 16" - Ladies 18" and Mans 19" . Your Stroller is a 18" 3/5 reed with a beefy
construction with quality components. They were made in 18" and 19" keyboards and in Red, Blue, Green, White & Black.
The Black, White & Red the most popular.
These early Strollers and even with the later Victoria made models were extremely popular in the US as in the time dry
tuned 3 reed quality made accordions were with their L & M sets of reeds for popular US music of the time and the
L M H reeds used for polkas & ethnic music. They were lightweight & had a volume of sound for strolling. Amplified
they were used in many small groups