Furnace help

For all of you past HVAC personnel… What is the opening for on the left side and/or what’s supposed to be installed there? Concern? BTW, it’s a Trane XE, manufactured in 2000, I believe (someone could confirm the age as well, if you don’t mind - SN=R462YCS1G)

Thanks in advance.

looks like an inspection hole. No screw for the cover?

Thanks Scott!