Looking at your photos again the spring is bent (that's why the tab on the top of the photo is sticking out) and w/o the spring it won't work properly.
Judging by the type of spring I see you likely need this
http://ccspecialties.org/pics/ptokitb.jpg
PN/ IH-543690-R92 Use 759-3490
I found it here:
http://ccspecialties.org/ptocreepers.html
the button you need is P/N- IH-473483-R1 (OEM fiber) a lot of people have gone to the bronze ones, but I've never had an issue with the fiber one.
that page has a whole lot of stuff for mechanical PTOs. Good luck.
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Daniel G.
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(May 1970) 147 w/an IH spring assist, 48" deck, 42" blade, 1969 73, #2 trailer, 10" Brinly plow and (on loan) Dad's #2 tiller.