This is the widest lens available from Mamiya for the TL/DTL series. It wasn´t until the MSX/DSX series until Mamiya came out with some wider lenses for the M42 screw mount (a 21 mm and a 14 mm fish-eye).
The earlier silver TL version of this lens has a 50% larger minimum focusing distance of 0.45 m (1.5 ft.), Otherwise, the optical design is the same.
Compared to the SX version of this lens, the DTL version is quite a bit longer, the diameter is about the same. The optical formula has also changed between the TL(DTL version and the SX version. I haven´t had a chance to compare these two lenses yet.