They began stamping numbers on the frames in December 1925, four months into the 1926 model year, so early 1926 frames will be unmarked. If it has the wide rear cross member (26-27) but no number, it's an early 1926 frame made in late 1925.
As the other guys said, the number is usually on top of the right frame rail under the floorboard. But sometimes it's on the left rail. You may have to apply some steel wool and clean off decades of crud to see it.