Spiegel had 2 special configuration scopes made exclusively for them by Mossberg, Gary had an example of the one with external adjustments at the last fest, I recently acquired one with internal adjustments. I don't know if these were the intended scopes for use on the Spiegel specials, the one I have has very short eye relief and eye position seems very limited, but both have mounts that allowed the scope to slide forward and came with rubber eye cups.
The 1940 catalog pages I have do not have the 99 but they do list several 22 caliber Stevens labeled rifles and list the Stevens 29 (actually 29-A action) as drilled & tapped for scope blocks and available in a combination with a scope (which is why I bought one of the scopes).