The tee is shown on the parts diagram.----Diagram shows one outlet from primer valve to be capped.----Perhaps the primer valve was replaced and someone was confused by the simplicity of this valve!
There was a service bulletin in 1994 to move the tell tale fitting to the top of block.----So 27 years ago.-----The latest factory water pump kit has bits / pieces to make cooling system modifications.
???----Just take the lower apart and re-assemble.-----Use grease to hold the yoke in place.----Be less work risk than drilling out a drain plug hole !!!
#1 and #3 were both hot.-----This motor needs a compression test done first.-----Or inspection as already stated.-----If pistons are loose / rings bad it will not generate enuff crankcase compression to start when submerged.---Simple as that.
Lotions a potions do not help. ----A bump with a hefty hammer is needed.----Or tighten the three screws a bit more till it pops.-----The locking tapers are holding it.