Doin' some research I'm coming up that these roofs were still exceptionally rare during the steam era... Can other SPFs confirm? They'd be very unlikely to be paired with a coach that didn't have the three stripes...
One of my books shows that ~140+ class P70fR were built in mid 1930’s: the one pictured in that timeframe has an arch roof. So even if 30% of the class had arch roofs, they would be very likely be running behind steam, IMHO.