Porcy,
Great pictures - especially the Atlantic.
Atlantics were built with tenders with both D-shaped and oval (the last 10 Zs) frame slots.
As you say the depth of the angle iron below the footplate is no guide, as these were altered in some cases to suit the depth on the Atlantic locomotives.
Our (NELPG's) Q6 was completed in December 1918, so had a tender with oval frame slots - so we can say with confidence that the current tender is not the original and that, though the tank is new, it matches the proportions of the one at withdrawal. No tender of those proportions (I do not say exact dimensions) particularly of height of tank, with continuous coal rails around the tender rear, originated other than on NER Class V, V09 and Z Atlantics, except for the 10 R1 Class tenders, which all seem to have gone to J39s.
So I think it is very likely that the current frames on 63395 are from an Atlantic.
Small consolation for the lack of a Z in preservation.
Look out for a change of livery in 2238/63395's Centenary Year!
Regards,
Roy