Yes, they're known as dovetail groves, and are 11mm...as opposed to a Weaver/Picatinny which are around 20-22mm
As mentioned above, 11mm is the standard size of scope rings, etc, meant for most air rifles and .22s.
Of course, there is also a 11mm - 22mm conversion plate - but it would start to look a bit much on the small Henry, although I have one to mount red dot on a 900 fps air rifle.