The 90º hole has a standard set screw in it, which is wrong to start with. Set screws for couplers have a square head
with a wire hole thru the head
. The hex socket set screws can come loose and fall out, allowing the prop shaft to slide out of the coupler just at the worst moment (Murphy's Law).
As for the end, if that hole is threaded then that bolt
will not go in all the way due to the unthreaded portion of the bolt, unless part of the threaded hole is counter bored deep enough to except the shank of the bolt.
INHO I would pull the coupler off the v-drive/shaft to see if the prop shaft end is threaded and put in a sq. hd. set screw (sample picture below but with a safety
wire). Actually there should be two, either at 90º or 180º.
The end hole could possibly be a jacking hole to force the shaft out of the coupler and the bolt came from somewhere else or is a lost
Take a mirror and look around under the trany/motor for an empty hole.