1. Moore's made marked items with the ship transfers in the 1850s
2. Moore's made similar looking plaques in the 1850s
3. There are very few plaques attributable to Scott from the 1850s
4. There are very few bowls marked Moore from the 1860s
5. The majority of transfers on brown-bordered plaques originated at Moore's
6. Scott's transfers don't appear on brown-bordered plaques
7. The lustre decoration on jugs and bowls with ships matches Moore items
8. The ship transfers never appear with Scott's flowers
9. Moore's was a bigger pottery than Scott's
If we put aside the decoration of plaques, jugs and bowls with ship transfers, we still have a clear idea of what Scott's was doing in the 1860s. They potted a large number of jugs, ewers, and bowls, and they decorated many of them with their own (group 1) transfers, often with typical flower decoration. They made their own particular rectangular plaque form, and decorated those plaques with the group 1 transfers.
But what else was Moore's, the larger pottery, doing during that period? If you subtract the brown-bordered plaques, and decorating items for Scott's, very little remains by way of lustre items. However, according to Baker, it wasn't until 1875 that Moore's 'concentrated chiefly on making dinner sets of various patterns, abandoning what it considered the old-fashioned transfer-printed pink lustreware'.
Moore's had the capacity to make the brown-bordered plaques, and to take in pottery from other factories for decoration. Aside from doing that, it's unclear what else the pottery was producing, by way of lustreware, during this period of technological improvement and expansion.
10. Records show that Scott's sent items to Moore's for decoration
The group of transfers that Baker is referring to are those that appear on brown-bordered plaques. There's no evidence of sharing of Moore's or Scott's transfers from the 1840s. And there's no evidence of Scott's sharing its group 1 transfers, until c1870, and the orange lustre period.
I'm still waiting for a response from the Sunderland Museum regarding the records Baker referred to. However, regardless of the date of the records (if they are indeed dated), I believe, for all of the reasons above, that there's compelling evidence that Scott's sent items to Moore's for lustre decoration in the 1860s.
Thanks to anyone who has read this far. If you have evidence against my reasoning above, I still am, and always will be, happy to consider it.