pamtnman, the Holland and Holland bullets were, afaik, meant to be greased instead of paper patched. H&H was pretty early in supplanting the PP bullet with GG bullets.
I don't think there is much worry in shooting a smooth sided solid PP bullet made of any reasonably soft lead alloy as the taper is so gradual.
I'm pretty sure the topic of the Lancaster oval bore was discussed somewhere along the line and the minor and major diameters were not too great, maybe a few thou I think.
A chamber cast will reveal a lot in this case. For sure, the case neck and bullet will be perfectly round. The leade will be interesting to see as to how it transitions from round to oval.
Also, eventhough the bore is oval, you can still use a pin gauge to measure the minor diameter and to measure any taper the bore may have.