Because my SEIKO 6306-7000 has faded bluish bezel insert, I immediately chose the blue one to wear on the 6306. After wearing it for two months, now I can safely say that this is one of the best rubber strap I ever have.
My previous rubber strap is the SEIKO OEM DAL1BP 22 (a JDM version of the classic flat vent Z22). With thickness around 3.7 mm, William's MM300 Type II is thicker than SEIKO OEM DAL1BP 22 or Z22, but it is softer and more comfortable on the wrist. It curves nicely on my wrist (around 6-1/4 inch) the first time I wear it, without the need for "hot water and cup treatment".
I have tried some silicon straps which are very light and soft but they somehow feel fragile and have unpleasant stickiness on the skin. This MM300 Type II have the best of both worlds: thick solid rubber which gives the feeling of ruggedness and security, but also soft and curves nicely, even on small wrist.
For more information about William Jean's Marine Master Type II 22 mm Rubber Strap, check his website: strapboutique.com. More pictures of the straps below. Thanks for reading.