I will not tell you that it is not the genuine article. That would be
unethical and foolish of me, but I will share some information with you
to help you judge your course of action.

Since well before the turn of the twentieth century, manufacturers of
violins, often nestled in the valleys of southern as well as western and
eastern Germany, were producing tens of thousands of violins annually,
labeled "Antonius Stradiuarius, Cremonenfis Faciebat Anno 17 " and
had a circular embellishment on these labels which included a cross
above the initials "A" and "S". More often than not, the last two digits of
the date were penciled or inked in by hand. Sometimes it was left blank.

These manufacturers, housed in such towns as "Bubenreuth", "Mittenwald",
" Markneukirchen" to mention a few, mass produced
these violins, in part by hand or completely by machine, and, until 1957, 
labeled them exactly as the master did. After that date, the words
"Copy of" were often included on the labels.

They were also made world-wide in such places as Bulgaria, China,
Czechoslovakia, France, Japan, England and elsewhere.

Millions of these instruments exist today. Antonio, along with his sons
are believed to have made more than
1,100 instruments during his lifetime of which roughly 512 violins are
thought to still remain, depending on who you're asking.

It is also known that many were destroyed either by fire, accident, lost
at sea or floods, during the fire-bombing of Dresden, etc., which leaves
virtually none unaccounted for.

To determine whether yours is the genuine article you need to take it in
person to an expert near you.
These experts may and often do charge for their time and opinion.

The odds that you have found a genuine Stradivari will be very slim indeed.

Nonetheless your find might be compared to an oyster; -
"if you don't find a pearl inside, you can still have a good meal".
In other words, make beautiful music with it.

Respectfully submitted