That'd have been a first not to have both wings convoyed on the same convoy... My bet is that they replaced the planned 1Wing on Convoy16 with the RearFuselage part of that new MSN, and here would be the rest of the frame... But it would be nice to have this confirmed indeed
I would say it denotes some industrial transition. The last time the same frame was carried on two different convoys was when the A380 wing factory was reorganised in order to produce improved ('fixed') wings. That's why I believe what we are seeing now is part of the transition to a lower rate.