Hi Matt
Having done much research on these verts, thanks to fig, who told me the "bug" would bite after I bought what was meant to be a 1971--but is in fact a 1960 convertible, here is my 5 cents worth:
Yours is indeed a 1958 car. The 115A and 115B series were only introduced in 1965. All VINs before that date carried the same numbering as a Beetle non-convertible.
Your dash knobs should be the ivory colour for 1958, like the other one shown in the pics above.
You would have had semaphores in 1958. It looks like these were covered over. Knock on the "B" pillar behind the door opening and you will hear a thud on the upper part, as aopposed to a hollow knock where they would have been located. Semaphores in most markets ceased in 1960.
You can get 12 volt semaphores from
www.justkampers.com for about 170 pounds. You must use bus semaphores as they stuck out further than the Beetle ones did, as they were placed lower down because of the top and were harder to see from behind when lower down.
Your front fenders would therefore not have had the indicators on top--these would have been added after.
Your bumpers, front and back, for 1958 would have been the blade type with no overiders, although these are now acceptable for safety. Ha ha.
Your steering wheel is obviously incorrect. It would have been the flat two spoke, with the spokes off set under the horn ring. You may have to get a new rod and spindle as yours may have been adapted. Also semaphores have a difficult wiring conundrum for the turn signal.
The front fenders should be flared at the back where they meet the running boards--I cant tell from the pictures. If they are not, you possibly had later fenders with indicators fitted.
They fuel gauge is an after market addition. You would have had the reserve rod and switch. Your tank would have been the humped on, and always painted the same colour as the body on the Karmann convertibles. Painting when you get around to it had some very detailed specifics you will need to look into, like hinges, seat legs, shifter, etc, etc.
You should have a 1100 34 HP (maybe 36 HP?? I can't remember) engine--if you have it with a serial number between 3 098 191 to 3 152 758, then you most likely have matching numbers, and you park the car for your pension!!
You would have had a split "crash" gear box with no syncro.
It would have been 6 volt as you know.
I am sure there is much, much more ..........
I enjoyed this as I have spent days in research to get mine right, and in the process have tried to understand what changed where and when through the years--I just don't know where to start on mine!! Two months ago I thought a Beetle was a Beetle was a Beetle............
James
must admit im a little "green" 
