Never had mine apart so can’t help there.
I did place it in a pan on the stove to know where 212F is early on.
Now that I have a thermal gun (they have become very reasonable and great for checking temps of exhaust runners for weak cylinders, rear diffs in tow vehicles and trailer bearings too) I have found the mono meter to be very accurate. It never sees actual coolant flow. But it does see the highly saturated air/steam of the coolant. You should leave an air space above the core. I can now after years, look at the mono meter and based on ambient know when my coolant level is getting low.
When slowly filling a low cold radiator, listen for when the core ‘bubbles’ this is the point where you have just reached the top of the core and have covered the tubes so they bubble with coolant and escaping air. Then add a ~1/2 gallon more once the bubbling stops since you have filled the core and you are good but not too full. It’s tough to see in there to get a good full level otherwise due to the flow deflector plate and overfilling just spits it out the overfill tube when running and people yell at you that you are boiling over.
The see thru window of the mono meter is to alert you to the red and also to be seen at night in oncoming lights. It works.
Like me, many of you have other vehicles. I keep a log of when I change their coolant on a rotational basis. I put the still good used 50/50 from one of them in the Buick to protect from freezing in the Fall for storage and during the touring season just top off with water. Repeat each season.
Another lesson on storage. Many old engines have a ring of mud around the base of the cylinder water jacket. Draining just water can leave this ring wet and there have been cases where this ring freezes and cracks the jug in what was thought to be a ‘dry’ engine. Always store with 50/50. Or is a minimum soak that crud ring with some 50/50 if you have been running just water.
Also the foot from a pair of pantyhose hose makes a great filter to temporarily place in the upper hose and secure with the engine out tube hose and clamp. Crazy the crud you will collect.
A leaking water pump shaft seal can and will suck air at cruise speeds causing lots of entrapped air in the flow and bad cooling performance. A very common issue. ‘Hey my leak goes away above idle!’ No, you are most likely sucking air.
I blew 50 dried bug carcasses out of my fins with an air hose. Who know how old they are. Had to help.
Pay at attention to fan rotation and pitch. You wouldn’t be the first or last guy to put a fan on backwards or wire an electric one backwards. Idles fine and at cruise speed the two air flows (ram and fan) match and you get no air flow. I fixed 3 over the years that were brought to me with this issue after the owners gave up. One guy only did parades for years because that is the only speed it would cool well. Now he drives it like he stole it.