A thought occurred to me so I compared four different rubber bumpers. Per the attached picture:
1. The front suspension rubber bumper C6OA-3025-A is the original question - I found several with the part numbers, others have found an F.O. designation, no part number.
2. The 8" pinion bumper with a C4 embossed part number.
3. A triangular shaped rear axle end rubber bumper, no part number
4. A square shaped rear axle end rubber bumper, no part number
So it is a mixed bag, no safe conclusions.