This picture from Paul Bateson shows Southend PD3 337, CJN437C, operating the first service journey under the High Street railway bridge on route 29 on Sunday September 26th, 1965. The roadway under the bridge was lowered so that the PD3s would fit, and judging by the queue of traffic behind 337, the driver wasn't taking any chances.
At the time, Paul worked in the traffic office at Southend Corporation Transport - my boss when I worked there as a conductor in the summer two years later - and he now lives in Canada and jointly runs, with Roger Hardy, the British Buses Abroad in North America website

Click here for fleet details.

Photo taken by Paul A. Bateson, Sunday 26 September 1965, Southend High Street