The rear sight style had nothing to do with the DX versions.

When the Classic models were introduced in 1990 they were drilled and tapped for scope mounting, not so with the standard 29 and 629 partial shroud models at the time. A new rounded front rear sight was put on the 629-3 and 29-5 Classic, Magna Classic and Classic DX models at this time.

When the 629-4 and 29-6 were introduced the rear sight was standardized to a new shorter version round front rear sight style (still in use today).

Those early long/rounded front rear sights are hard to find as they were only used for a short time.