A/C vs. Non A/C fire wall

Does a non A/C firewall have a different opening than a A/C fire wall. Im curious because I tried to mount a non A/C box into my A/C car and it wasnt even close to fitting. Then found out that there was a trim ring for the fire wall side that was used to support the non A/C box. This was on my 71 Scamp. I would love to see pics of the mounting, or bare fire wall with either option box.