Neat bench!
Yeah, the grooved crossmember is for a board jack; is the top grooved on the underside, or are there signs of a separate piece that was removed?
I'm going to guess that rather than a hardy hole, the square mortise was for a "board stop" for use while planing surfaces. The stop could be removed, or hammered up and down to adjust height (you want it on the end of the board, but below the surface so your plane doesn't hit it). The two round holes are almost certainly for hold-downs. The mortise and holes are both in the right (i.e., "very common") place for that use.
Age? If the bench I started on had been stored in a barn for the last 30 years, it and this bench could be siblings--or your bench could be 150 years old. ("My" bench was made about 1960.) Nothing that shows in the pictures suggests a pre-Civil War date.
You're probably going to have to start the search for toolmarks, and then it will tend to be more of an exclusionary factor. If you find circular marks from the sawmill on the bottom of the benchtop, for example, it couldn't pre-date the introduction of the circular sawmill. Based solely on what i can see in the pictures, I'd guess no earlier than the third quarter of the 19th century.
Look at the head of the vise screw, and on the garter hardware, for a name--there is a fair chance the vise screw at least was from Ohio Tool Company and will be marked as such.