First off, the shell plate really does look like it was made by Star. The G is a puzzle, but I have seen some odd markings on Star shell plates.
The slot where the case is inserted looks like it has a beveled edge which would be typical for rimless or semi-rimmed cases that are held in the shell plate by the extractor groove. Shell plates for rimmed cases like 38 special and 45 Colt are held by the rim and the slot is not beveled (so far as I have seen). This doesn't prevent it from being used for 45 Colt if the case fits and the recess on the bottom of the shell plate is big enough to accommodate the rim, and is the correct depth.
The diameter of each caliber shell plate is different. This makes it possible for the shell locator straps to push the case all the way into the shell plate.
I wonder if it was a 45 ACP shell plate that was modified to fit 45 Colt. I have seen such things. I won't name any names.
Bruce Williams