Just a wild shot in the dark, not knowing the hymn, but I'd guess that this is notation from a chord organ book, and the numbers represent the notes, with '1' representing the bottom note on the keyboard. Judging from the notes, I'd guess it is not a C, because -8 would be a Cb, which doesn't make a lot of sense.

I'd guess: -6 Eb 6 E 7 F 8 G

Boyd