Stan
According to period catalogs the Single Sighting Plane rib (noted in Hunter Arms option code references as the SSP or SP option) was not available until about 1938; so your gun predates this option. That said, the barrels of your gun could have been replaced with a set having that option; although such is unlikely as Smith guns I've examined with the SSP rib option are taller at the standing breech in order to accommodate the additional rib height.