With a casting date of A-3-8 (January 3rd, 1968), a stamping of 06-15 (June 15th, over 5 months later), and a QX code that doesn't appear to be a factory stamp, (not to mention that it's not in the correct format)... well, let's just say it doesn't follow the normative practice. However, I can't tell from the pictures if the axle tubes are tapered (factory JL8) or not (service JL8), so if you can take a good shot of where the axle tubes meet the backing plates, it might help. If they're tapered, there's no way it's original since the factory didn't release the JL8 for production until February of 1969. If they're not tapered, then theoretically it's possible since the service (over the counter) JL8 axle was released in March of 1968.
Ed