Hi again RoadRunner,
The easiest way to determine the year of the bike would be with the wheels...but it all depends on how genuine the machine is. 1956 and '57 Golden Flashes used the same engine and frame numbering prefix and these were fitted with the "Ariel" type alloy full width hubs.
For 1958 models BSA fitted the cast iron full width hubs so this is the first thing to look for when determining the year. To change the front wheel would mean changing the front fork legs to suit so it's not a simple job. Fuel tanks often got damaged (or leaked) and were easily replaced so I wouldn't go by that.
The bike in question could be a '56 or '57 model even though, as you say, it shows 1958 in the log book. The log books show the date of registration, not the date of manufacture, and the bike could have been stood in a showroom for several months before it was sold. Hence, a '56/57 model registered in 1958.
I have a B31 that left the BSA factory in February '58 and wasn't registered until March '59. For years I thought it was a '59 model!!
Hope this helps.
Beezageezauk.