RYU ... the "stock gear set" shown in your first post is the countershaft assembly (page 13.29 of workshop manual).
I'd really like to now it's weight, if you could through it on some (good) scales please.
As to whether its USDM or not, I suggest count the teeth on the 2nd gear cog (the largest one), then someone will chime in with teeth count for USDM and JDM.
And if anyone knows the weight of the mainshaft assembly and differential moving parts please let me know...