According to Willie's list
http://veloce.se/STAT.HTM last 105.57 (LHD non US models) had chassis 1411787 and was build in 1969/70, first S2 1750 had same type ID (105.57) but 182 prefix to the chassis numbers, first built '69, ch. no. 1820001. My 1750 S2 have ch.no. 1820173, was built on 4 Sept. 1970, according to the Archivo.
By the way, yes, there is some confusion of the numbers produced of at least the 105.57. If you count the numbers built every year you get 3036, but the Total states 4039????
Roundtails (S1) was, according to what I've read, built by Pinifarina and S2s were built by AR in Arese, so production can have overlapped untill parts at Pinifarina had been used up. For instance, last RHD S1 (105.58) was built in July '70, first S2 built in April '71.
These cars often stayed at dealers for very long time, so many roundtails wasn't sold to custermers untill 1971 or 1972 - in Europe they were regarded as very expencive, they retailed at the same price as 2 MGBs, so buyers didn't exactly que-up at dealers show rooms to buy Spiders.
Erik