I think more than one train has been cannibalised. GWR say they have 15 175s but all the figures say 16 as does the Train database.
They only require 19 (the 19 769s) to run a full service and release the 158s. The other 7 are spares or local enhancements. 3+2 trains, 2+2.
Cannot see any 175s today.