can some one tell me what goes here?

[degree01]

Hello, i have a 1966 chevy truck that i am restoring and there is a hole about 3" across that is under my heater/air system and i have no idea what goes here of it is part of the heater unit, but on the heater unit there does not seem to be anything that would go there. Here is an attachment of the picture of that round Hole and if some one can tell me what went there, I would appreciate it.

thank you all in advance.