Probably a mix of 3 ply and 6 ply drums.. Rounded blue/olive badge? I have seen that in the past.

I would think that the 3 ply would be pointy badge and maybe the 6 ply as well if they were in the transition period.

I don't think it's any stretch that in 1977 or 78 when they were switching over to 6 ply shells that they shipped out some mixed sets. At the time no one was really paying that much attention to such things (least of all Ludwig). Serial numbers on factory kits can be different from each other by thousands (pulled from a parts bin and randomly applied) so that doesn't really indicate anything.

Pictures of the kit?

I've not ever heard of a "mixed plies" set sent out by Ludwig.

Mixed badges I've heard of but not mixed plies.

I actually know someone with a mixed ply set. It's a 20,12,13,14 in bowling ball oyster blue. The badges all have close serial numbers, the wrap is a perfect match on all three drums and two of the drums are three ply while two of the drums are six ply. The set has a factory double tom mount and it looks like it was ordered this way. It's set up in a studio that I recorded in once.