V_Jaytest is a weapon that was used by Metro Cops, 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 really 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.
The model however is used as a placeholder when used by Gasmask Citizens, Combine Elites and Metropolice.
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.