This building is recognisable by the domes along the roof.
It isn't there in photos of the racecourse from the 1910s, but can be seen in photos from the 1920s.
I guess it was built in 1919, as part of a scheme to build permanent stands to replace the matshed stands that had previously been built in this area of the racecourse. There was a major fire in the matshed stands in 1918.
Moddsey found this in section 41 of the PWD Annual Report for 1919:
The erection, on R.B.L. 33, Happy Valley, of four large stands for the Hongkong Jockey Club was completed early in the year.
It isn't 100% clear they're referring to this stand, but it seems likely.
If you can give any more information about it's construction / demolition dates, or how it was used, please let us know in the comments below.