You are correct it is the proof mark is for polygonal rifling (5 sides)
Not all glock have it because the 45's and 10mm don't have polygonal, rifling they have six sided rifling because the larger diameter bullet needs six sides to give the barrel more contact with the bullet. H&K is the same way
Pentagons, hexagons, and octagons are all types of polygons with different numbers of sides.
All Glock barrels have polygonal rifling but none of them have pentagonal (5-sided) rifling.
The polygonal rifling in Glock barrels is either hexagonal (6-sided) or octagonal (8-sided) rifling depending on the model/caliber.
Not all H&K pistols use polygonal rifling. I have one of the early H&K USP pistols and it has standard rifling.
Glock's polygonal siding is subtle. It's more rounded than others. That's why it won't look obviously plygonal. But if you collect a bullet fired from one it'll look much more flat sided than out of a non-polygonal sided barrel.
Correct. It's not simple to tell that it's polygonal from looking down the bore but you can tell either by looking at a fired bullet, or by looking at the profile of the rifling where it intersects the crown. The latter is especially telling when it is done side-by-side with a barrel with conventional rifling.