Anodised & etched on aluminium. There seem to have been a number of versions of these plates over the years, and of course they are not detailed in any of the parts books. It would appear that this design was used on both 750 and 101 series Giuliettas in some markets. The best method to determine which type was correct for your car as sold in the original market is probably to search Google images for engine bay photos of original cars supplied in the market where the car was originally supplied or sold. By "market", I am thinking about countries (such as Italy, Germany, USA) which are known to have had specific regulations or requirements to display particular information about a car on the ID or similar plates.
 
Part Name: Chassis Id/Data And Lubrication Plate, Aluminium, 20 X 8 cms, Photo-Etched
Alfastop Product Code: 1ID100
Original Alfa Romeo Part No: 
Model: Giulietta 750 & 101
Quantity Per Car: 1