That is a beautiful crankset! That particular model was produced from around 1954 and made well into 1970s. The Stronglight crank arms with that particular lettering were typically found in use as original equipment on many different bikes. Seldom found in such perfect condition. But, unfortunately actual value is probably quite low. If the bike is in comparable condition I would use the crank on your bike!
You could actually mount the single 42t chainring alone directly on the 3-arm spider.
The BCD of the 3-arm crank is probably 116 mm. and if you are patient you might find larger single chainrings to fit.
DOH! ... spoke too soon! ... here is one on eBay right now which would be a beautiful match and would be VERY practical for your needs too! -->
Item #140263809026
... 1/8" ... chrome plated steel ... 46 teeth ... sounds perfect for a fixed gear or single speed 
The axle is probably the British standard diameter of 5/8" at the ends... and with cotter pins 3/8" (9.5 mm.) ... and I would suspect the cranks even have Standard threads for 9/16" x 20 pedal axles. A shorter axle could be found or just try reversing the left and right sides to check your chain line for use with a single chainring. The typical Stronglight cottered axle total length recommended for use with a single chainring would be around 123 to 127 mm. ... but if the reversed axle is adequate to clear your chainstay just use that one and simply leave the left side crank arm sticking out a wee bit more.
