The left Crest or 'Coats of Arms' is the original Crest when Slough was the biggest and richest town in the county of Buckinghamshire. Slough became a Chartered Borough in 1936. The motto Fiducia et vi means By confidence and strength. source
The right Crest is from 1974 when Slough was transferred from Buckinghamshire to Berkshire. The Labour Party removed it from council stationery and public building signs in 2004.