I would have to concur, the last few covers seem like an after though, both the bird and journey series - its like someone just pull a note out of his/her wallet/purse for the mock up of the cover just before it was printed, although I'm sure this is not the case. No doubt painstaking hours of meetings of Charlton Catalogue Cover Selection committee have when into the selection of the appropriate note to each catalogue.
The original 1984 Catalogue actually had 2 or 3 notes on it, I'm sure if they did that now it would add another $4 to the issue price