To my knowledge this had always been the Belfast Water Commissioner's Office but closer inspection revealed a series of letters set into the wall. At first I thought the letters were I.N.R.S. & O. and tried in vain to match sensible words to the initials but the best I could come up with was Irish National Rivers Something or Other but a bit more research produced the fact that originally this was Richardson Sons & Owden's Linen Warehouse and further research reveals the letters are, in fact, J.N.R.S. & O.
So, with M&S now ensconced in the building, it has almost reverted to its original purpose.
The M&S building can be seen in the background of this pic and if you look closely you will just see the above letter to the right of the two young lovers.
