There are two slightly different metal covers for the light switch body. The earlier style does NOT have depressions on the top, one on each side of the hole for the wires. The later style does have the depressions. Both of these covers have the same part number from Ford. The 48-3647 bail and the 78- bail need the cover with the depressions. The depressions hold the bail in place. Because the earlier style cap does not have depressions the effect is the cap is slightly taller and the later style bails have no place to lock into. The early cap uses the B-3647-B bail, as pictured above post #29, which is pre 1935. The 35-39 cap with the depressions is shown below left.