Mat­tey was also one of the few man­u­fac­tur­ers will­ing to pro­duce stere­o­scopes on request for a wide vari­ety of stereo for­mats. This is con­sis­tent with a com­pa­ny sup­ply­ing oth­er busi­ness­es rather than sell­ing direct­ly to end users.
(from: Stere­oscopy History)

The 9x12 for­mat, though rarely seen with stereo­scop­ic images, was very com­mon for mono pho­tog­ra­phy. Pho­to­graph­ic plates and their equip­ment could be adapt­ed eas­i­ly for stereo­scop­ic pur­pose by using a stereo­scop­ic attach­ment / lens board with a stan­dard cam­era. This indi­cates that there was a mar­ket for ama­teur pho­tog­ra­phers tak­ing both mono and stereo photos.