That's a very nice machine.

I agree with Uwe about the transition to single pane glass in the latter half of 1923. The very first Royal 10s (late 1913-1915) had the two rectangular cut-outs in the side of the frame, but Royal did not put glass in them until late 1915.  A couple of other differences that I've seen in the transition is the switch from the large Royal decal, as you have, to a smaller one on the paper table. The early dual pane models (pre 1918?) have large articulated mechanical paper guides. The very early dual pane 10s also have elegant pin striping that gives them a beautiful distinctive Art Nouveau look.



The paper guides, concurrent with the dual to single pane glass, made a two-step transition from the articulated mechanical paper guide to the lighter gauge, ruled paper guide that attached to the top of the paper table. The transitional paper guide (maybe 1920-21?) between the two variants was attached at the bottom left of the paper table and did not have a ruler.

I have examples of both and will post them if you are interested.