Ok, *hopefully* this might help:
3 examples, 3PO, R2 & a Yoda. two are production material, one is a custom cast, can you guess which one?
The C-3PO is a Hardcopy Prototype from the Action Masters line, which was worked on around a similar time as the Blue Harvests started to appear on the market.
This is an actual prototype production piece.
The R2 is a Blue Harvest. It is a cast in production materials used at Kenner, but from the vintage line using old molds.
The Yoda is a custom cast, mostly probably from a Blue Harvest. This is a totally different material and clearly shows airbubbles marking the surface. Both examples are from the unproduced Micro Collection Dagobah playset, but one is a cast from the original mold, the other is cast from the Blue Harvest cast.
Although Blue Harvests are not prototypes from the production process of pieces at the time they were createed, they are cast from the production or test molds. Blue Harvests of production pieces in most cases are 1:1 blue examples of the final piece, however, as mentioned before and seen above, there were also a number of unproduced examples created, which are either at different scale, different sculpt or unreleased, which offer a tiny glimpse of the production process to create something.
Hope that helps.
Dave.