Exactly Gus! Consider that when the light "Bobs" were reconstituted in the 1770s, many were issued pattern 1760 light infantry carbines set up in 1761 and 1762. Complaints of "bad" carbines issued to the light infantry were very common but it was the only dedicated light infantry arm available. Given the discussions in this thread, I can almost hear in my mind the debate going on about side plates for the new pattern 1769 muskets. The warrant called for the convex plate as Bailey documented but I'll bet John Hirst argued "Hey, I've got a lot of these flat side plates in stock from marine and militia muskets not set up. They work just fine and cost less." Or perhaps a contractor negotiated with ordnance to use the flat, and perfectly suitable, plates to cut his cost. Hence, the pattern 1769 had a flat side plate and not what was specified by ordnance.
dave