You are right, Wendl & Lung and Hailun are identical, except for the soundboard stamp and the fallboard I believe.
One word that can explain pretty much about your doubt is 'stencil'. http://www.pianoworld.com/ubb/cgi-bin/ultimatebb.cgi?/topic/1/24513.html#000000
Here's a thread started by me, a discussion about home vs stencil (more specifically Hailun vs W&L).
Try to search up the word 'stencil' and you can get a lot of information. Just in case you don't know, stencil means pianos made by someone else but bear your own fallboard name. For instance, Hailun makes piano for W&L, but it bears W&L on the fallboard instead of Hailun.
There's many reasons about this stencil contract, part of it is that a Chinese manufacturer (let say Pearl River) wants to establish their names at a certain market but the name will certainly steer people away. Hence, they use a name (let say Smith Bros.) that doesn't sound like a Chinese name to open up their market. Once their market is established, they use back their own names and establish their own reputation.
A very good example is Steigerman Premium at the US which is manufactured by Hailun. I read somewhere that Steigerman Premium is phasing out its production already.
Hope it helps.