It's a "tweener!"
After carefully comparing the typically available MC30 schematic (M152-D xformer), the earlier version that Terry posted (M152-B xformer), Mickey's comments, and the chassis I have (whew!), it seems mine falls in between the later and earlier versions. Specifically, the chassis I have matches more closely the schematic Terry posted, but it does have a convenience outlet. In addition, some of the cap values match the later schematic. Just makes it all the more fun!