That's where the hole for the steering lock was on early bikes. I think the hole was used through '65, then was indented until maybe around '68 when it went away completely. I don't have firm years available but I think the indent may be correct for your '66.
There are two Spares manuals for 1966, early and late versions. The late version is almost identical to the 1967 model year manual. This is probably why there is some confusion regarding parts for the '66.
1967 BSA Wasp 1967 BSA Hornet (West Coast Model) 1967 BSA Hornet (East Coast Model) 1968 BSA Firebird Scrambler 1968 BSA Spitfire Mark IV