After studying the above responses, and poring through the Compendium, I would agree that it was originally a Model A that was later 'Amberolized' and converted to work without an end gate at some point.
I notice the chamfer at the end of the upper works (beneath where the end gate would have latched) which does not seem to appear on Model Bs or later; and I detect a slight remnant of the pinstriped spiral nearby.
The 'shims' under the upper works are perhaps replacements for the rubber insulators that preceded the spring suspension system…
Why the end gate was removed is a bit of a puzzle: perhaps an earlier collector found it badly damaged, and could not find a replacement, and so made the modification?
I know this is turned in late, so I don't expect a grade...
Bill