Oldsmobile-88
Very well known Exeweb poster
That would have been Talfryn Thomas(1922-82) who played Private Cheesman after there was a spare position in the platoon after James Beck(1929-73) suddenly died.Phillip Madoc along with the Welsh chap who was in Dad's Army for a while (Can't remember his name) were the go to people if they ever needed a Welsh person in a TV programme back in the day. Madoc though was a good actor with that strong, deep voice, Either a villan or a person in authority.
Thomas was only in one series(1974) he had a major part in Survivors(1975) as Tom Price. His part in subsequent Dads Army series was not reprised.