At least 78-82. Catalogue pages here:

http://bikecatalogs.org/SCHWINN/MODE...n%20Sport.html
http://bikecatalogs.org/SCHWINN/1978...LL/1978_29.jpg

If the Twinn uses the same system of SNs, I /think/ SN should be on the left rear dropout with the form XX#####, where the second letter is the year (A=1965, B=1966,...) and the first letter is the month (A=Jan, B=Feb,...). Letters "I" and "O" are skipped.

http://www.angelfire.com/rant/allday...innCodes4.html