The bridgeless bottom bracket shell and internal brake cable routing are indicative of a Montello. The helical ridges in a SLX seat tube only extend about 150mm up from the BB shell and they're only 0.35mm thick by 3.5mm wide, so I'm not suprised you couldn't see them looking down the seat tube. Try looking into the BB shell with a strong light or stick your finger in and feel for them. If you want a reference for what you're looking/feeling for, the same ridges are used inside the bottom of the fork's steering column.