Depending on when in 73 tractor built,tow/push valve could be in 2 different locations on hydro pump due to fact sundstrand changed over to a piston pump design & a new manifold on same in mid 73,hydro pump may be older style used to mid 73 or newer style pump.
With fender/seat raised & looking at pump from pulley side,look at manifold mounted on rear of pump,look at left top corner of manifold,if pump pre 73 design you'll see a stub shaft in left corner with a roll pin thru it to form a T handle(roll pin may be missing)turning this shaft 1/2 turn will put hydro into freewheel.
If no stub shaft in top of manifold then pump newer piston style(after mid 73)got to opposite side of tractor to see rear side of pump/manifold(left side of tractor)look at left forward faceing side of manifold,according to service manual at very bottom of forward faceing side there should be a stub shaft in manifold(may/may not have roll pin in shaft)this is tow/freewheel valve.As has been said there should be a decal on frame/trans with an arrow in it pointing to valve,but it may have come off.
Goto website below with mod/sn of tractor off id tag,see if service manual for transaxle avail as a free download,also check if operators manual a free download.
CAUTION-IF you start fooling with trans & have to add fluid on pre mid-73 pumps trans fluid is ATF dexronII(red)if pump mid 73/later fluid is 10-30 or 10-40 SAE motor oil,do not mix the oils.