I suspect that the SN list is incorrect, as my son's 1970 BS (with dual lens side markers) is SN 25735.
It doesn't make sense that there were ~11000 1969's built, ~800 1970's, and ~25000 1971's. Could it be that the "1971" 1438's were actually 1970's? This would seem to add up correctly, though there were probably (as others have stated) a few late SN cars that were titled as the following model year.
Thankfully, parts for Spiders are (relatively) interchangeable.
1977 Spider 1800 (SHELOB - driver)
1970 124 Sport Spider (99% complete barn find, now in my garage, awaiting restoration)