I got you an answer over at the http://www.allaircooled.com forum:
Hello,The ashtray you have that is Chrome was a stock item in 1970 that was added to the Bug by the dealer as an accessory or came from Germany as factory installed package.
The aftermarket black ones are copies of the Mexico and Brazil newer models and have a deeper tray. (I think because they used the Bugs as taxis there)
Anyways, the chrome ones in the rear panels are correct (black stock in dash) You can use the black ones with modifications.
Hope this helps.