The Original shocks that Bird used for their bikes was a brand called Sebac. The eye to eye length was 11 3/4" and use 3/8 or 10mm bolts which was very common in the mini bike world in the 1970's due to distributors like Azusa that supplied american manufacturers with parts importing and carrying them here in the US.
I have never seen spring rates listed unfortunately, but they are good combination for bikes of this size. Some of the universal shocks offered today that come in around the same size have a much stiffer spring rate and also sometimes have clearance issues (note the tapered bottom for instance). If you run a set of those new ones on a light suspended seat, make sure you get a saddle with really good padding :wink: Also remember that shock length is pretty critical to the way the seat will sit so mounts may need altered to make the seat sit right.
Kinda dark in my garage this early in the AM but here's what they are/were: