I also have that first production of the 870, the ratio was 4:1. The most annoying thing from this old reel was actually the lever drag, the drag quadrant was very short, what I mean is the range of the lever drag. So it was like either on or off. I called Matt Harper and he sent me a replacement right side plate, complete with the gears. I asked for a 6:1 ratio. It was all in there, even the drag cam was supplied and of course the pinion gear. Oh, even a longer handle arm was also included.
Later I also bought the cage with the cast control. Practically everything is new, next to the spool and the old clicker system
Unfortunately I also don't have the schematics diagram of this old 870. The number of bearings is the same, but the old one has smaller o/d anti reverse bearings. Clicker system is also different, old one still uses the Platinum style clicker, not making so much noise as the new plastic clicker. Mainly, very few plastic parts on the old one. You can easily use the newer version's schematic for servicing the old 870. But if you plan to use it a lot, do consider to change the side plate with the wider drag range. It is a lot easier to use and looks also nicer.