it is Rosso Alfa 501, but... se below
I compared several color samples from different brands before deciding on what I belive was the factory shape of red used on my spider. As you can see above, the Maxmayer A.R. 501 Rosso Alfa 21/B (left) matches the Lechler 505 Rosso chiaro, whereas the Lechler 501 matches the Maxmayer 501 21/A.
It must be considered that for the same color there are different shapes. The A.R. 501 Rosso Alfa changes over the years, so it is darker on early cars (21/A) and a bit clearer since 1960 (21/B). My spider is a 1960 101 interim Veloce, then I chose the 21/B from Maxmayer. if I had used Lechler products, I probably would have chosen their 505 Rosso Chiaro.
As a side note, I will refer what an expert told me about the processes of painting at the pininfarina factory. He discovered this curious fact while, using advanced chromatography techniques, he was trying to establish the right shape of each early A.R. nitro color in order to reproduce them with modern chemicals. Shortly, there was no color equal to itself! More precisely, only the cars painted together in the same batch were the same color, that because, quite simply, the tank cleaning between the coating of a batch of cars and the other was not a priority at that time, so, if, for instance, before painting your car they had painted a batch of white cars, your car was a lighter color, or darker if the previous batch was black and so on.
Frankly not sure how much of this is entirely true, but something tells me that things were not going very differently.
One thing is clear: often, to be faithful to the factory specifications, we become trapped into dilemmas to the limit of absurdity, and only a look at how things really were in those days shows us that we risk being more catholic than the Pope