I have been a member here for a long time but don't post often... hoeever, I DO read!! Here is what I found on your "new find"..:
SINGER SEWING COMPANY
"AD" Series Register Numbers
Factory Name: Elizabethport
Elizabeth, New Jersey
Register Numbers Class Quantity Date Year
Letter From To (Model No.) Allotted Allotted Allotted Notes
AD- 205531 208530 96 3000 August 26 1930
AD- 208531 209030 147 500 August 26 1930
AD- 209031 209130 57 100 September 3 1930
AD- 209131 209630 114 500 September 3 1930
AD- 209631 212630 31 3000 September 23 1930
AD- 212631 215630 95 3000 September 23 1930
AD- 215631 225630 99 10000 September 23 1930
AD- 225631 225730 124 100 September 23 1930
AD- 225731 235730 128 10000 September 23 1930
AD- 235731 255730 15 20000 October 14 1930
AD- 255731 256730 32 1000 October 14 1930
AD- 256731 286730 66 30000 October 14 1930 :idea:
And here's where I found it..:
http://www.singerco.com/support/serial_numbers.html
Click on the 'Double Letter Prefix' link in red!
Hope this helps out some.. If I find anything else I will post..! 8-)