Vortex Venom footprint, or also Docter footprint or Noblex footprint, is a footprint standard that you can find used with mainly full sized red dots.

You can recognize this footprint by few details:

• it has 4 sockets - one in each corner,
• the front & rear sockets are 20mm far from each other,
• the left & right sockets are 39mm from each other.

If you have classic handgun, you need to be searching for a red dot mount which replaces rear sight in the dovetail cut: red dot mount with Vortex Venom footprint:

• Red dot mounts for Beretta models
• Red dot mounts for Canik models
• Red dot mounts for CZ models
• Red dot mounts for HK models
• Red dot mounts for Glock models
• Red dot mounts for HS Produkt models / Springfield Armory models
• Red dot mounts for Sig models
• Red dot mounts for Tanfoglio models
• Red dot mounts for Taurus models
• Red dot mounts for Walther models
• and other

Make sure to search for mount with "Vortex Venom footprint" that also has your handgun in the compatibility list.