AFAIK, all the factory "thick" sheetmetal receivers were made to the same OD as the thin ones, therefore the trunnions and furniture were narrower to accomodate them. Some of the US made thick receivers were made to the same ID as the thin ones to accomodate normal AKM parts sets instead of just the narrower ones. These are the ones that require the longer pins and bending of the topcovers to fit. When the Russkies went to the AKM they kept parts specs as much the same as they could, which is why they have stepped rails to accomodate the carrier groove for the milled models instead of making a new carrier spec that would only fit the AKM. Less spare parts to inventory and issue.