Chris, what you've got there is the earliest production type! Normally the bulged canopy appeared about the same time as the variable pitch 3-blade airscrew, so your comment is right - it would originally have had a Watts wooden 2-blade screw. Modifications appear to have been made as new parts became available - I've seen a row of Spits with both flat and bulged canopies and three-blade screws. Other things to note are the original type of TR9 radio mast without the shroud, and the twin prong pitot tube under the wing. Done up like that, it's typical of early-mid 1939 - talking of the paintwork as well.