What is a plowman trundle?

I saw this sign on the side of a building in Bridgwater, Somerset; apparently it was built in the early 1800s and is Flemish-bond cream brick, with stone coping It has twentieth century additions and maybe the sign should actually read ‘W.H.Smith, Plowman and Trundle – Saddler’ – I think! It is a little confusing because there is a large stationary and book retailer called W.H.Smith with branches right across the county, including one down the road from here in Bridgwater.