Maybe you're right.

When were the 28" barreled, 3/4 stocked 'Afrika' Mausers manufactured?

Scopes, or 'telescopic sights', were popular add ons for Mauser and Mannlicher Schoenauer rifles and stutzen prepared for British and American trade from at least the early 1920s. Didn't the 28" barreled Type L (lang) 'Afrika' Mauser predate that?

Notice that in the image from above post that the rifle closest to the chap in his Sola Topi appears to be one of the long Afrika Mausers. Notice the 3/4 length stock and length overall compared to the other rifle visible in the image.

For Mannlicher Schoenauer the '39 Stoeger lists, among custom options, barrels up to 26 inches but not 28. The 10.75X68, however, is not mentioned. Could they have been longer on special order? 'Off the shelf', they are listed with 24" barrel as are the other 'High Velocity' MS.

'Factory' scoped MS mounting was actually contracted out by Steyr to several Austrian guild shops / craftsmen. They tended to be of the 'Vienna Snapper' type (as shown above) or claw mounted.