As you note, Bianchi's Superset II is a mixture of two tubesets. Superset was introduced for the 1984 model year, which was also the last model year for Piaggio, so your bicycle would be a 1984 model. The serial number will provide confirmation on the model year. Bianchi stated that the 1984 Limited used Ishiwata 022. I'd assume that this is the primary tubeset, as it this would be the most advantageous approach from a marketing standpoint. The down tube would likely be heavier gauge Ishiwata 024. However, since 017 though 024 all used the same 0.8mm chain stays, the Superset chain stays would be custom drawn. To the best of my knowledge Bianchi did not use heavier gauge tubesets on the larger Superset frame sizes. This can be confirmed by the seat post size, would typically be 27.2mm if 022 was the primary tubeset.

So, assuming an 022/024 mix, your frame should have 0.9/0.6/0.9 top and seat tubes with a 1.0/0.7/1.0 down tube. The fork blades would be 1,0mm. The seat stays would be 0.8mm. The chain stays should larger than 0.8mm standard used on 017 though 024. I'd assume custom drawn 0,9mm, as their 0245 used 1.0mm and it was hi-tensile.

An Ishiwata 022/024 Superest configuration would be comparable to a Tange #2 /#3 Superset configuration. Unfortunately, Bianchi did not identify the Tange tubesets used on the later versions of the Limited and identification is further complicated by the fact the Tange used a common seat tube for their #1 , #2 and #3 tubesets. An Ishiwata 022/024 Superset frame would be virtually indistinguishable from a Tange #2 /#3 Superset frame of the same size and geometry.