This is HUD only version which can be also refered as "light" version of Classic Zombie Panic! Source.

The only changes here are edited HUD materials to old versions, in hopes to make thing classic\old school enough.
Below is a full list of changes:

• Old health and armor icon.
  
• Old voice communication menu(X and C).
  
• Old infection icon.
  
• Old zombies icons, including horde health icons.
  
• Inventory icons, mostly based on old icons but modernized to 3.x version a bit.
  
• Old background for menu. Uses different font, as I dont have original font.
  
• All textures got "No level of detail" flag.

If you are a looking for core version of Classic ZP!S, you should click on this link, so you can get old viewmodels, textures and more.

And for question that probaly will be asked a lot:

The main menu or other textures are still the same as it was. Fix plz.
I cant do anything much about it. The issue is laying down in mounting process causing some materials elements simply not refreshing. For example, background menu. When you launch the game and when you are in main menu, the game starts mounting process. Once the mounting process is done, the game simply do not update materials, resulting the same 3.x menu showing up. To fix this, you have to open console and type in:
The game will freeze for a mere second or so and once refreshing materials process is compelted, you will get every texture and old background working.

Alternative way(Better solution if you have 3+ more mods installed).
Place my addon in zps_custom folder.
To do so, head to Steam > steamapps > workshop > content > 17500 > 1452116710. Copy everything from that folder and navigate yourself to zps_custom folder. If you dont know where, here is the path:
Steam > steamapps > common > Zombie Panic! Source > zps_custom. If such folder do not exist, simply create one with that name. After that, paste files into that folder.
REMEMBER
Whenever this addon gets updated, you will have to repeat this step again to apply changes, otherwise, some changes might simply not work because of priority system!