This looks like a 1928 set offered in a 1927 Proof Set Box.

[ozjohn]

 I have a 1927 set in an identical box even down to the remains of tape on the back of the box. I don't know if any 1928 proof sets were ever issued but this looks like a cobbled together set of  1928 coins presented in a 1927 proof set box to trap the unwary at a high price.

[ozjohn]

3 hours ago, ozjohn said:

 I have a 1927 set in an identical box even down to the remains of tape on the back of the box. I don't know if any 1928 proof sets were ever issued but this looks like a cobbled together set of  1928 coins presented in a 1927 proof set box to trap the unwary at a high price.

Further investigation suggests that the 1927 proof set was released in the cardboard case for 15/-  per set with and additional 6/- for a set housed in a leather case.

[Peckris 2]

My own set is in the cardboard case. Cost me rather more than 15/- though...