Ed had requested in a thread on another site so rather than taking that one off focus I thought I would share here Unfortunately I don't have a collection of all of these parts so I have to rely on my collection of pictures instead
Because of where the car was being shipped, optioned or how the original buyer choose a very few Mustangs each year received no heater.
69-70 Heater DeleteThis first picture is of what was described by the owner as the block off plate (sort of round plate) the plate retainer (strap in the lower left that would go across the opening on the interior side and be held in place by the center nut and stud on the plate) and the black heater control panel. Can't confirm or dispute at this point
This is reported to be a 70 heater delete plate
Engine side
Interior side. Like the earlier plates it seems to install from the interior side and be held in place by studs and nuts through existing holes in the firewall
An original in place on a 70 NJ car (possibly an export) Note that it looks like a PO added a section of plumbers tape to stabilise something they added - possibly a CB or underdash 8 track player