Your transmission should be a Spicer 210-24 Series Model 8844A, with a 8 qt oil capacity (40 weight motor oil). The lower plug is the drain (referred to as a G3 in diagrams), and is likely a 3/4" pipe thread, torqued to 45-55 ft lbs. The fill plug is probably a 1 1/4" pipe thread (referred to as G2 in diagrams) torqued to 60-75 ft. lbs. No dipstick, and I don't recall a level plug (but it's been 25 years since I serviced a Spicer), and I don't see any reference to it in any of my manuals from that period. You should be okay at this point. Failing getting the (maybe) level plug out, work your finger in the fill hole to see if you can reach oil, or feed in a tube or wire to find the level. There no level plug fill it till runs out lol wait till you check the drop box those are fun to fill