De standard 20mm PU-strap of the old Sumo (orange SBDC005). I do not see a difference between this strap and the standard strap of the SKX013, onlu a different Seiko coding. So if you are looking for the most original replacement for your SKX013, this strap is the one to choose. Except for the SEIKO on the short end, this strap can also be used on an old model Monster. Both the SKX013 and Monster (SKX779/781) straps are discontinued. But as you can see, there are a lot op 20mm diver strap options at the moment.
This is a polyurethane strap, so it is hard and not so flexible. It will get the round shape for your wrist in time, but you can speed up this process by putting it (made round as much as you want it with a piece of string or so) in really hot water. The width is 20mm between the lugs, the “shoulders” are 23,1mm and it tapers to 18mm at the buckle.