Been searching for possible leads on your post, however, there's nothing that would seemingly point towards the manufacturer are you looking for. It would also be hard to say that this particular China manufacturer is the official manufacturer for H&M since there are so many of them in the market.
Also if you have read through the other posts on the forum, most if not all members (especially the senior ones) and staff will discourage or warn you about buying anything branded and expecting it to be authentic, if your main source is from the East. Although we are not saying that all Asian suppliers are bogus rather just be warned.
Just by going through the H&M site, it seems that they only sell either through their own stores or online (but limited to a certain number of countries only).
Is there anything else that you are interested in? We'd be glad to help you out :)