I have guessed his “canon” choice of pistols based on menu images featured in-game.
About the appearance of ICA19, I considered the H1&H2 version and the H3 version the same. And while the one featured in H1 locations looks closer to Black Trinity, you could argue that technically Black Trinity didn’t exist until H3 and the one in those pictures is just beta ICA19, so I wrote both names.
Expand to see the basis and the “alternative”. Texts with * mean no evidence could be found within the location and was speculated based on related cinematics or out-of-game materials.
ICA Facility: ICA19
The only available starting pistol for the location.
Paris: ICA19 or ICA19 Black Trinity
The image for the challenge “A Break from the Auction” features the beta design of ICA19.
Sapienza: ICA19 or ICA19 Black Trinity
The image for the challenge “.45 Therapy” features the beta design of ICA19.
Marrakesh: ICA19*
Primarily featured in promotional materials.
Bangkok: ICA19*
Primarily featured in promotional materials.
Hokkaido: None/ICA19(Patient Zero)
No pistol can be brought into the mission without Mastery Levels.
(Alternative: ICA19 or ICA19 Black Trinity - The image for the challenge “A Personal Goodbye” features the beta design of ICA19.
)
Patient Zero: Featured in the mission image
Hawke's Bay: ICA19
The original HITMAN 2 immediately starts the mission with the default loadout(ICA19, Fiberwire, Coins) upon the first start.
Miami: None
No pistol can be brought into the mission without Mastery Levels.
Isle of Sgàil: Unknown
New York: ICA19 or a black variation of it*
47 assembles what seems to be a black 1911 pistol in the briefing.
Dubai: None
No pistol can be brought into the mission via the default starting location(new game).
(Alternative: ICA19 - Featured in “Waiter Cater” challenge image.
)
Dartmoor: ICA19 Black Trinity/ICA19 Chrome(The Dartmoor Garden Show)
Featured in “Death In The Family” mission image and various challenge images.
The Dartmoor Garden Show: Featured in the escalation image (I don’t think what we do in it is canon, but I still wanted to include it)
Berlin: None
No pistol can be brought into the mission via the default starting location(new game).
(Alternative: ICA19 Black Lily - Featured in “Apex Predator” mission image and various challenge images.
)
Chongqing: ICA19 Silverballer*
Primarily featured in promotional materials. And it fits the default suit and the location the best.
Mendoza: ICA19 Chrome*
Featured in the following cinematic.
Carpathian Mountains: None
No pistol can be brought into the mission.
If you have your own opinion or found something I have missed, feel free to leave it here!
Edit: Added images and more guesses.
Edit 2: Polished the post. Added Ambrose Island.