The B/O badge dating info shown here and other places on the 'net shows 1969 through 1972 production by individual year, then shows 1976 using badges 917xxx through 1290xxx.
I take this to mean that 1973 started somewhere around 917xxx and 1976 ended about 1290xxx, with no information on 1973, '74 or '75.
Doing the math, and assuming equal production for the period, I came up with these ranges for starting serial numbers by year:
1973 917000
1974 1010250
1975 1103500
1976 1196750
1977 1290000
This is just a guestimate, but I haven't seen anything better yet.