I appreciate your time and the information you provided. Some very useful information that I hope can show me the way here. I’m almost certain that I’ll need a shim to make this right. Thanks so much for passing this alongI am probably out of my league here, but I often get more modern ('50s-'80s) racing and touring bike frames where the post is incorrect. It is usually smaller than what the frame would initially have used. My experience has been that the slot should have each side being roughly parallel after tightening when using the proper sized post. I have regularly received frames where the slot is pinched together more at the top from the use of a post too small for the bike, aside from those frames where the clamp area was out of round though usually from using too small of a post. Every frame I have fixed where I made the slot sides are parallel has had a proper size post fit without and clamp down without slippage.
That slot appears larger at the top which may indicate that some had pried it apart to remove a seized post. It may be that you have the correct post but the hole has been widened from previous efforts. Again, I am not all that familiar with these older bikes and their eccentricities.
I would certainly be cautious and if concerned at all about damage then use a shim. I have intentionally used a smaller post before (while not having or waiting for one), but I never clamped down on it where the slot was pinched at the top. I also have never had a post clamped into the frame where the slot was wider at the top with the exception of previous damage from being pried open.
I use "Shim In A Can". They have brass, stainless steel, steel, and copper at various thicknesses. I regularly use the brass shim for many uses without any issues thus far.