third member numbers

I see a lot of third members on e-bay for sale, and the I keep seeing 3 numbers being repeated 742, 741, 489. What do these represent ? I have a 73 swinger I would like to upgrade to a 8.75 (will be relocating springs inward), are these a good deal and will they fit into any size 8.75 housing. I've seen housing with measurements of 59 1/8" and 56 1/2" end to end. whats better find a complete rear end or piece one together , Hope some of you can help steer me right