Alfa Romeo used a 3 digit model ID system, which is the first 3 numbers of the serial number. each new chassis type received a new number. first cars after WW2 where 750's. Wheelbase stretch becomes 101's, the 105 began in the mid sixties, suspension redesign and use of frt crossmember. later the US versions of the 105 series where labeled 115 to designate the EPA/DOT modifications. the GTV/Guila coupes and sedans from 65? to 74 in US and 76? in Europe, and the spider, from the Duetto thru to 1994 are the 105/115 models. 116 is Alfetta thru GTV-6, Milano, followed by cars that used the 3 digit system as their names. 157 164 etc. I guess technically the only 105's in the US are the early 1600's the S4 spider is a 115 in the US, BUT still a 105 everywhere else. Most owners consider them all as 105's, but a few owners will point out that doing so is incorrect.