The D&RG 1923 Equipment Roster lists only car N.G. under the 7xxx sequence, a Flat 7447, all coalcars 1000-1925, 8168-8760, 8800-9574 with some 8 cars listed with 97xx and one 9939. No listings for the idlercar either. D&RG did have (1890's) Flatcars with Sideboards numbered in the 75xx and up, photo appears in the N.G.Pictorial V-iii but these in no way resembled a standard Hi-sider. DPL have photo showing a Flat 7121 with one board on stakes CHS.X9565 but again way back when....
I looked and looked again, under extreme enlargement, the numeral does take on the appearance of a seven but would have to be the skinniest seven ever.
So I would conclude that the spacing would indicate a
1 since the
7 should fill the same rectangular space as the
0,
3 and
2.