Wich year of construction...

...has my Gilera 150 Tourismo of the 50ies?
Can anybody help me?
The serial number of my bike is 186 - 5344
The year is unknown because I bought the bike without docs
on an italian scrapyard 20 years ago.
So now I have to search the year of construction
to get german documents. Gilera says they have no more documents of this time.
So if someone has a serial before or after mine and knows the year of his bike,
i could reduce the period. It must be build between 1954 and 1957
(The aluminium exhaust was only build at this time).

Tanks!
Joh