This is the rarest of all SOHC outer timing covers - the pre-1938 'Ribbed' type outer timing cover, but made in magnesium. As far as I am aware, there were only a handful of these covers made, for the 1937'ish Big Plunger Works race bikes.
These covers are copied from an original and look exactly the same. So, if you want to give your pre-war cammy that most exclusive look - they do not come much rarer or unusual than this! - they are also manufactured in high qualiity AZ91E heat treated magnesium alloy - so are suprisingly light when compared to the more normal alloy version of the same cover.
Note: in the photos these covers look slightly wet. This is the protective water repellent oil I spray them in before placing them in a plastic sealable bag. 3\16" BSF screws should be used for the rev clock fitting. Update November
01/03/2020 Update: We are pleased to be able another small batch of these magnesium alloy outer timing covers - this ribbed variety sold out very quickly when we last offered them 6 years ago. However, sorry to say, the cost of magnesium alloy foundry costs has risen out of all proporting to normal alloy since that time. We only use the best quality foundry (who cater to the motor racing and aerospace industry) - and all machining is by our regular CNC partner. After machining we then fully re-chromate them - before spraying and packing in Duck Oil, so although not cheap, we believe they are a high quality item, reflecting that cost.