I'm cleaning out a tool box tonight and came across this tool. It's marked : caution  wear gloves  Made in U.S.A.  PAT. Pend. The the two adapters are marked : 3/8-24 and M10-1.5 . They are also slotted . I know I've seen these before but can't remember what it's for. Any ideas ??

Huh, no idea.

Perhaps for loading a spring ? Belt tensioner?