Level 0 Vintage
For Vintage items, the old level system used to be 0-100 with but was later changed to 1-100.
There are 6,721 know instances of level 0 Vintage quality items, and 324 known instances of level 0 Vintage Ambasador.
Source
Vintage Ambassador
Team Fortress 2 Level 0 Revolver
Crits on headshot
-15% damage penalty
20% slower firing speed
No random critical hits
Critical damage is affected by range