P.S. - the socket depth problem was the bulb on the far right side of the panel, just outside of the "100" mark on the speedometer.
I replaced the bulb above the ammeter (which also illuminates the right side of the tach and the left side of the water/temp gauge), but could not get to the matching bulb, illuminating the left side of the speedo, petrol, and right side of the water/temp gauge. Maybe I'll try again next winter when I have more time....