Una, I just spoke with the world's best SA at my local H&M. She knew immediately what shirt I was describing. It's from their Trend line. Only the flagship H&M stores carry that line. (She herself has over 300 of their Items, she confided.) Closest flagship store is San Francisco...

But, she said to check first thing online every morning, as stuff does get restocked in the Trend line. Also, the Trend line will be having a sale
beginning a week from Wednesday.

Hope this is helpful, and good luck!!