I found this info from renotalk.com :
Below is a quick summary of the popular brands:
King Koil in Singapore is manufactured by Matsu****a Great Wall, they are a licensee only, they don't even use the same logo as King Koil in USA.
Simmons in Singapore is owned by a Japanese company, Simmons in the USA recently went bankrupt and was bought over by another company. Simmons in Singapore is overpriced because their technology is out-dated, you can get the same kind of mattress for half the price by buying a reputable local brand like Four Star.
Serta in Singapore is manufactured in Batam, in the US Serta sells Mira Coil or continuous coil mattresses in Singapore they promote pocketed spring mattresses.
Slumberland sells the same kind of spring as Serta in UK but in Singapore they try to pass it off as pocketed coil as well.
Sealy in Singapore is owned by Sealy Inc. of the US, the product is the same as the US but it is manufactured in Thailand to reduce cost.