RESIDENT EVIL 2
44 Achievements
1,070
27-33.5h
Customizer
Customize a weapon.
10
48.89%
How to unlock the Customizer achievement in RESIDENT EVIL 2 - Definitive Guide
Once opened the desk will contain a high-capacity magazine which can be combined with Leon's pistol.
I believe the puzzle and codes are the same for Claire and she gets a speed reloader for her revolver.
As far as I know this is the earliest place in the game to get this achievement, at least without cheating.
Throughout the game you will find upgrades for all of the base game weapons and they will be found usually behind locks, in safes, or in otherwise initially inaccessible areas. The first weapon upgrade you will likely acquire can be found in the safe in the 'Waiting Room' that is on the second floor of the Police Station on the East wing as you are looking at the map. The combination for this safe is 6-2-11.
Once you have this upgrade, it will be different depending on whether you're playing as Leon or Claire. Go into your inventory, select it, then combine it with the weapon it belongs to. If you are using the suggested upgrade then it will be either for the Matilda if you're playing as Leon, or the SLS 60 if you're playing as Claire.
There are many more upgrades throughout the game and any will do for this achievement.