Firewall oddity , dbl thick on right side

[72 K20]

I noticed this on my 72 C20 today. The right side of the firewall is double thickness. It appears that the second skin was put on to correct the openings for the air conditioning. The truck was originally Ochre,, the inside still shows the original paint on the firewall. The spid shows air conditioning ,, the vin plate still has the correct rosette rivets.

Look at the pictures and tell me what you think took place.
Have a great day !! Tom