I was rather surprised to read that the author from that referenced article from the 1990's (from that information starved, pre-internet era) claimed to know the number of Diana 58 specimens manufactured. I decided to see for myself what that author actually claimed in that article:

" Next in rarity is the second model, though the sheer mass and quality of this airgun makes it second to none in terms of desirability. I have no data on the numbers produced, but judging from the lack of specimens encountered, and considering what should be an abnormally high survival rate for so fine a piece, one could not imagine more than a few thousand at best."

Oh...that is what he actually stated!