I think Glen Gower was under 1600grt, so probably only R/T. Sorry I can't answer your question about the callsign until next time home on leave, where I have a 1958 Lloyds Register. I went on day trips with my parents from Portishead and Clevedon piers on Glen Gower, Glen Usk, Bristol Queen and Cardiff Queen many a time in the 50s summers, often returning to Hotwells in the evening from Barry or Penarth.
My 1963 call sign reference book records the call sign of a Glen Gower as MWFC. That ship had a registered tonnage of 15 Tons, was registered in Greenoch with an official number of 163765. Not sure if she is the ship you are interested in. Hope the info is of use to you.
A quick search on the internet shows me that Glen Gower was 553grt. Last year in service was 1957, so I would have been 6 years old or younger when I was a passenger. The older single-funnel Glen Usk lasted a couple of seasons longer.