Well, the only Martlets in British service in 1940 were the 81 ex-French machines plus another 10 ex-Belgian contract machines that were taken over by Britain. The French machines had been modified to use 6 x 7.5mm machine guns that were to be fitted in France. After taking over the order, the Brits reinstated the 50 cal armament. It's likely that conversion work was done in the UK by Blackburn's (typically, aircraft delivered from the US were bought without armament, which was fitted in-country).
Perhaps, in the conversion process in the UK, Blackburn was able to come up with a wing gun installation that worked? Regardless, we're still talking about less than 100 aircraft...so suggesting that it's a "considerable improvement" is perhaps stretching things a little in the 1940 timeframe?