What bullet weight was the HP designed for? 115/124 or 147 gr.?
I'm thinking one of the latter two, but not sure and am curious as to which weight it was supposedly registered or calibrated to fire.

I don't know what it's designed for but the C series and MKIII pistols I have prefer the 124 gr. bullets.

The 147 grain is a latecomer in 9mm ammo, originally for SEALS to use in suppressed SMG's.

Either of the other weights would have been used in 1935.
I think the original spec, 1902, was for 124 grain truncated cone bullet at 1040 FPS.But by WW I, there were varieties.

I owned a couple of HP's and both functioned fine with both 115 and 124 grain bullets. The MK III guns have stronger slides and fare better with heavy use of Plus P ammo. But most people don't need to shoot hot loads a lot.

That's my preference in bullet weight for the 9mm.
I don't think the 115 was even used when the HP came out, was it?

The Brits and the Canadians load 115 gr rounds at 1200 fps for their Hi Powers during WWII.

Just based on how long the design has been around I'd say the 115g bullet.

Funny you ask this-

I was trying different loads in my MK II the other night. There wasn't a huge difference at 45 ft between the 115 and 147. Less than I expected, anyway.

Off a bench, I can usually see the distinct groupings, but for some reason, with that pistol, the POI was only an inch different at most. Wonder why?

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Because of the metric system?

The standard 9mm Luger Load in 1935 when the Hi Po came out was 124 gr. Material shortages in WWII brought about the switch to 115 gr.
There is not enough difference between the 115gr and 124g POI at combat ranges to matter.
The 147gr trajectory is different, BUT the added weight and slow speed of the 147gr over the 124gr causes the barrel to rotate upward more as the bullet goes down the barrel, so the POI at 15yds is practically the same.

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Science plus Art

"Don't look a gift horse in the mouth."

Old Trojan saying.

+1 to what's already been stated regarding timeframes but in my experience with HPs (mostly T and C series, 1964-77ish), see which load works better for you as I have never been able to peg a preferred load between the 115 and 124.

So my post will be a non-sequitur.....here is a picture of my HPs, one is a 1990 model and the other 1970s I think it has been so long since I got it.