It turns out that some companies like Butterfly, Joola, Nittaku, and DHS are right now selling more than one type of completely different ITTF approved 40+ ball.
The Optimum 40+ is a seamed cellulose acetate (plastic) ball made in China. This is the only 40+ ball sold under Stiga's name. It is a re-branded version of the DHS 40+ ball that has been sold the last few years (and therefore different from the new DHS D40+). Joola sells a ball exactly like the Optimum 40+ known as the Super P. (Both were approved by ITTF in April of 2014). You can find many other companies selling this same ball for last ~3 years. So that is the answer to your main question. To be honest, most people do not like these very much.
The Butterfly 40+ ball most widely seen these days is called the G40+. It is seamed, made in Germany and is different from everything else out there. It makes a very unusual sound when it bounces. ITTF approved it in August 2015. In spite of high hopes, it has not caught on much.
In addition, Butterfly continues to sell a seamed 40+ cellulose acetate ball that is made for them in China by either DHS or (more likely) by Double Fish. That one will play pretty much the same as the Stiga Optimum 40+.
Xushaofa was one of the first two companies (along with Palio) to market seamless balls. It is made in China of a different material and a very different manufacturing process. Joola sells a seamless 40+ ball essentially identical to it known as the Joola Flash. Many other companies also sell re-branded versions of this ball (approved in summer of 2015). Most people like the seamless balls. They are durable, and have a nice bounce. Some people have worried a bit about XSF recent quality control, but most other brands continue to be very good. They are different from the Optimum 40+.
The latest development in 40+ balls includes all the ones made with a material known as ABS. The first one of these was the Nittaku Premium (made in Japan). Nearly everybody likes it but it is by far the most expensive ball. Now DHS is introduced one called the D40+ ball, and it is clear that very soon now many versions of the D40+ will be introduced by other companies. DHS is selling theirs for a lot less than Nittaku. It the future, these are likely to become the dominant balls on the market, which is a good thing because nearly everybody seems to like the ABS balls.
It is not clear what will happen with cellulose acetate 40+ balls like the Stiga Optimum 40+. People from top pros to ordinary forum members complain about their lack of durability, low bounce, lack of roundness and their cost. It is quite possible that in the not too distant future companies will stop producing them and we will see mostly ABS balls and seamless balls.
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