Housed in the old Custom House at the lower basin at Bowling. See the other side of the railway arches here NS4573 : Bricked-up railway arches. The wooden bollards are inscribed 'MM'.
This square reputedly contains the closest concentration in the UK of a navigible river (Clyde), navigable canal (Forth & Clyde), 'A' Class road (A82) and main line railway (West Highland). All within approximately 400 metres (was even less before the A82 was realigned).