The Jaytest (also known as the v_jaytest) is a model found within the Half-Life 2 Beta leak. The model is nothing more than a prototype model of the MP5K reusing the exact same sound files of the gun.
V_Jaytest is a weapon that was used by Metro Cops, Combine Elites, Gas Mask Citizens and Conscripts. The textures appear in
models/Police/ in anon-hl2, this shows that the Metro Cop model once had the SMG permanently affixed to Gordon's hands. The textures are similarly identical to the MP5K meaning the model is a very old model that was possibly removed.
It is concluded that V_Jaytest is a leftover model from from a much earlier developed era of Half-Life 2.
In certain maps some use the Jaytest while others use the MP5K. It is unknown if the maps that used the jaytest were just as old as the gun.
Differences between the back textures
Another fact to note is that v_jaytest, in its default state, uses the Metro Cop back it instead features the correct "back" texture.
An interesting thing about that Metro Cop back texture that it uses is that it is an earlier version of the one in the Metro Cop.
- https://www.youtube.com/watch?v=eiORL4QPadI&t=336s: Video of Metropolice, Combine Elites and Gasmask citizens using the Jaytest.