Hello Don. Your coin has a lot to recommend it. First, it has a neat pattern, with King Leopold III of Belgium on the front and a crowned set of nine shields on the back. Second, the coin is made of silver, with 0.537 troy ounces of the precious metal per coin. Third, there are triangle and cross variations that can really get coin collector juices flowing!
The large majority of these coins are worth Base Value (BV) plus a collector premium. Base value is computed by multiplying the silver content by the silver price. Today, with silver at $21 US dollars per troy ounce (look it up on kitco.com, it changes every day), the Base Value is 0.537 x 21 = $11 US dollars.
For coins in any condition above worn, add a collector premium as follows:
50 FRANCS DATED 1940
worn: BV (coin is worth Base Value)
average circulated: BV + $2 US dollar approximate catalog value
well preserved: BV + $10
fully uncirculated: BV + $25
The above values are for coins dated 1940, which are a little more rare than coins dated 1939. If you have a 1939 coin, divided the added amounts by two.
There are two numismatic (coin collector) varieties which warrant special mention. Some coins lack the small cross at the top of the crown. Other coins add a small triangle in the third shield from the left in the top row. Get out your magnifier. These varieties add value.
Here are the approximate catalog values for coins in well preserved condition. Lower grades will be worth much less, higher grades (fully uncirculated) will be worth much more.
Without cross: $40
With triangle: $60
Without cross and with triangle: $140
Hope you have the good one!