The list that you posted is a bit misleading because real funtion lost during the translation.
A heart-shaped cartouche centering an "I" is a marking of the shot tested main parts of the weapon. Usually before final assembly, the barrel itself as well as both the upper and lower side of the gun - in HP case the slide with the barrel, and the frame with the internal - was tested individually by a proof cartridge. This 'I' sign represents the successful test. This is an internal factory mark that was not mandatory to use.
I don't think so it's a roman IV due to Hungarian manufacturers and proof houses never used the roman letters as far as I know.
The HP's could have the following marks. The first letter in the mark category is the original FÉG reference letter about the stamp itself. On the drawing stamps placement marked with a reference letter in a circle.
Like this. This is an NGV assault rifle drawing. Sorry I have no available HP drawing now.
Category - PurposeB - Military acceptance marking
- 2.5 mm size K I. or K II. letter and below one/two 1.5 mm size number in a 5 mm diameter circle - Military Acceptance Mark (1951-1962), or
- 5 mm diameter, coat of arms of the Hungarian People’s Republic, and below one/two 2 mm size number stamp - Military Acceptance Mark (1962-1990).
Used: frame, slide
T - QC acceptance certificate
- 1.5 mm size D letter stamp in a 3 mm diameter circle, or
- 4 mm size D letter stamp - quality checked part.
Used: any part
P - Pressure tested with enhanced charge cartridge
- 1.5 mm size E letter stamp in a 3 mm diameter circle - Tested with enhanced charge cartridge.
Used: barrel
O - Tested with proof cartridge
- 1.5 mm size F letter stamp in a 3 mm diameter circle - A finished weapon tested by a proof cartridge.
Used: frame
K - Zeroed gun marking
- 4-6 mm size 0-9 number stamp - Indication of the accuracy grouping of the zeroed weapon. Higher number indicate lower accuracy.
Used: slide back side, near back sight
A - Final acceptance marking
- 1.5 mm size H letter stamp in a 3 mm diameter circle - The assembled weapon was tested, with standard (military) ammunition. Accepted for military and law enforcement use.
Used: barrel, frame
D - Ultrasonic examination marking
- 1.5 mm size I letter stamp in a 3 mm diameter circle - Part examined by ultrasonic device (typically the barrel)
Used: any part
Pl - Foundry / rolling mill marking
- 1.5 mm size two number stamp in a 3 mm diameter circle - Foundry or rolling mill plant identifier. In the FÉG case, this marking was between 01-40 refer to barrel forging plant.
Used: barrel
G - Hardness test indication marking
- 1.5 mm size G letter stamp in a 3 mm diameter circle - passed on hardness test, or
- 4-6 mm size 1-9 number stamp - tested hardness of a part
Used: any part
Component / HRC 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
Internal 1 2 3 4 5 6 7 8 - - - - - - - -
Barrel - - 1 2 3 4 5 6 7 8 - - - - - -
Frame, slide - - - - - - - 1 2 3 4 5 6 7 8 9
F - QC final acceptance certificate
- 1.5 mm size M letter stamp in a 3 mm diameter circle - Indicate the post-production quality inspection of a main part.
Used: barrel, frame
S - State test proof certificate
- 4 mm size, coat of arms stamp with N letter above it - Hungarian Small Arms Inspection Office - Nitro gunpowder test proof mark.
Used: barrel, frame, slide