Penn 1835 series corners -as found on many contemporary Peavey amps and speaker cabinets -such Windsor, Vypyr, Valveking etc. 1-9/16"x1-9/16" The screws are included with these!
This listing is for the C1835 3 hole corner, but these are also available in the 2 hole without lip version and the 2 hole with the lip which wraps around the inside edge of the cabinet.
These are sold in 4pc sets! Most cabinets will require 8 corners, but different style corners are often mixed and matched on the same cabinet. The 4pc sets will allow you to purchase the correct combination for your amp.