Very nice to see this revived after a few years.
I am an OPC collector. I can tell you that I have bought three or four factory sets over the years and five or six boxes, some of which I have opened.
If i remember correctly, the 'white backs' in factory sets were 'bulked' all togther at one end (I think the back) of the factory box. I'm going to guess that it was about 132 cards because that is, I think, the size of a sheet of uncut cards. This could constitute the 'entire set' of white backs but I don't know.
I will say that opening packs, both back in the day and recently, I have come across both white backs and regular backs in packs (what I mean by that is that the cards that have white backs also came in 'grey' backs in those packs... however I don't know if one box just had white backs or some boxes just has grey backs.
To me, this seems like the same thing that kind of happened in 1981 with OPC baseball. I can say this because I happened to chat with the head of early 80s distrubution while I was at a show in the early 90s (I live very close to London Ontario, where the factory was). He said, if I remember correctly, that because Hockey cards came out early in the year -- and because Gretzky was so popular -- they couldn't keep up with Hockey card demand so they had to dip into the card stock they had reserved for baseball cards in the spring. So, they ran out of white card stock and had to find a new supplier. That supplier only had grey stock (hence the 1981 late run Grey Backs). I don't know for sure but I suspect this was the case for 1990 as the baseball card market (thanks to UD and Griffey) was at an all time high and they may have ran out of the stock they had and had to dip into their non-sport stock, which was whiter than the first runs of baseball sheets.
I love this variation because after all these years, it is still challenging to finish without the price! Lots of folks don't know about it. I don't care about value, of course, but it is fun to go to shows and sift through 1990 OPC cards and try to find the 'DARK STOCK' of the cards because I believe the WHITE STOCK is more common than the normal grey stock for that one sheet (132 blue-bordered cards). Looking to trade, also.