It wasn't pristine as had initially been described, but it was in very good shape with but a tiny ding here and there but no rust visible anywhere on the bright blue frame or slide.
The pistol had been shot very little and cleaned even less.
I am not a fan of the old factory adjustable sights. In fact, those sights and their inability to hold zero for more than a couple of hundred shots led to my first custom Hi Power.
Still, it was a nice gun at a fair price so I bought it.
I pulled a like-new Mk III slide I happened to have (It had been matte blued but otherwise was stock.), and it fit the older frame very nicely. The bbl that came with the gun and bears the same serial number fit the Mk III slide quite well. Putting it into the slide and then pushing forward on the bbl and then rearward from the muzzle showed no movement that I could feel and the bbl's lugs were fully engaging the recesses in the slide. Checking this with a Blue Marks-a-lot confirmed what I already knew.
With the Hi Power assembled, the is no movement in the bbl-to-slide fit.
The bbl's feed ramp is the old humped version and if it causes any feeding problems, I'll alter it.
I stuck in a Wolff conventional 18.5-lb recoil spring, and knocked out the magazine disconnect. This improved the trigger a bit but not as much as I'd hoped for or experienced on other Hi Powers. I'd estimate this pistol's trigger pull at a clean but heavy 9 or 10-lbs.
Taking a few extra bbls I'd checked for fit in the gun, I grabbed some ammunition and slipped out to the range.
It was quite windy today and the longest distance I shot was 25 yards and did that while seated with my wrists braced. Did the same thing at 15 yards.
I did not bob the hammer spur as is my custom so I did get "bit" a tiny bit today, but I didn't want to alter the hammer if I wasn't going to keep the gun. The magazine "safety" can go right back in, but I cannot make the hammer spur grow so I'd held off.

Here's a picture of the Hi Power with the Mk III slide/bbl assembly in place. When this picture was taken, the magazine disconnect had not yet been removed.

At 15 yards, I blew 2 shots but only called the worst one. With the wind, the light hammer bite, and the heavy trigger, this was just about the best I could do. Others could no doubt do much better. The handload that was shot is not particularly accurate, but will usually do about 2 to 2 1/2" at this distance. I was shooting up the last of this mid-1100 ft/sec load.

At 25-yds, group size increased a bit and I still dropped one shot in the 9-ring. It was a called shot but not marked. The ammo used was Federal 124-gr. FMJ.
This Hi Power has the standard 32-lb mainspring and was used with the Wolff 18.5-lb recoil spring. These two standard pressure pressure loads worked fine although the handload didn't have quite enough "oomph" to lock the slide back once. The Federal ball as well as 115-gr. Fiocchi FMJ worked fine in all aspects as did a 124-gr. cast truncated cone handload, also over 6.9-gr. Blue Dot.
I am satisfied that the factory fixed sights are "on" so it looks like I'll be bobbing the hammer spur on this one. I think it is capable of much better grouping, but I'll need to work on the trigger pull to find that out for sure.

One of my other Mk III's has a bright blue slide with a matte frame. This one is just the opposite: bright blue frame and matte slide. For now, I'll leave the small factory single-sided safety in place as this one is a spare. When I get the trigger pull where it needs to be, I will take a look at fitting a slightly extended single-side thumb safety to the gun.
The way that bbls made during quite a span of years fit the Mk III slide and 1980 frame speaks pretty highly of FN's manufacturing in my opinion. Everything fit and only one bbl showed even a trace of "looseness" in the slide.
So, nothing real special, but sort of a neat way to get another Mk III and get to piddle around now and again to get where I hope to with this gun.
Besides, I just find it a heck of a lot of fun to shoot and mess around with Hi Powers.
Best.