The "p_usp_nosil.mdl" is a extra model which you can put it on your cstrike model folder. Remember, you need a specific plugin to see it on the game which you can find it on this descriptions (CS:GO Arm Switcher Plugin)

This weapon model requires a plugin in order to inspect the weapon that also you can find the link in the descriptions. (CS:GO Weapon Inspect Plugin)

Credits:
Valve - Animation, Models, Sound, Texture
CS PRO - Default USP-S model with CSGO animation
FunnkyHD - Compile Tool + StatTrak™ 
Viper - CS1.6 arms + CT sleeves
Pezot205 - CSGO-like gloves
Nextgenz & Dirp & irecon & FLaMmenWerFeR & Klint /Chillypowdah & Rene & sic - USP-S | Orion skin authors
aerosol - USP-S texturing

Don't have a plugin? Well, you can get the link below this text.

Plugins:

CS:GO Arm Switcher Plugin
https://gamebanana.com/mods/39783

CS:GO Weapon Inspect Plugin
https://gamebanana.com/mods/39857

Where I get the resources? Check this link below!

Resources:

CS1.6 Models | CS:GO Animations
https://gamebanana.com/mods/227227

CS:GO Gloves on CS1.6 Hands RELOADED 
https://gamebanana.com/mods/416233

CS:GO StatTrak Module
https://gamebanana.com/mods/287090

Notes: If I forgot to insert you in a credit, pm me on https://gamebanana.com/members/2501107.

Tutorial for newbies:

Copy and paste the file on: cstrike/models --> Model files
							cstrike/sound/weapons --> Sound files

Optional: cstrike/model/alt --> If you have a plugin installed. (CS:GO Arm Switcher Plugin)

Nonetheless, you have two model which is default version and sleeved version that you can choose from.