Just had chance to look at Douglas Centenary published in 1996, in it is a bit re M&S store in Strand St , the 2nd one (not the new one) which took over 18 and 20 Strand street by 1928 , 20 was Webb's lounge (dating from 1870's) , 18 was his wife's millinery shop (later Reids), in 1931 M&S took over no 22 and rebuilt them as one big store but no 16 it appeared was left as was, judging from a 1936 photo, woolworths was about 4 shops futher down (rebuilt 1951 after fire).
no 16 is 3 story - 1st floor has the rounded top windows with small 'pillars' separating them and very noticeable dripstones - when M&S moved out it was taken over by Boots, I think, but I cant quite recall from memory what the shop next to it looks like