DH fan, thanks for explaining what mirror means, which had bypassed me entirely. As you say, it meant that the standard masks were reversed rather than the colours. Intriguingly it was left to the various factories to how to turn out equal numbers of the A and B patterns, most choosing the serial numbers (ending in even or odd numbers) to be the deciding factors. Even when they were told to switch to one pattern only, it was left to them to decide if it be the A or B (most chose A).
The whole point of shadow pattern camouflage was to meld into the surrounding landscape. As aircraft were parked in a greatly and increasingly dispersed manner, any problems of a repeat pattern in a group of such aircraft were therefore remote, which was presumably the reason that the need for two patterns was dropped.