Looks like a toothed belt pulley to me. I've never seen a knob like that before, but then (other than Indian) I've never seen an hydraulic damper in that position either.
Single-sided damper Webbs (which is what this appears to be) were used from 1936-ish to the end of production, which was about 1947 or so. If you want the "real" damper knob then search on the various Velo or Norton web-sites, or check out Ebay.
Cammy Velo looking at it. Very nice indeed.
