Again, thanks for your input.
That FAA Order was superseded (recently). The new order is here:
http://www.faa.gov/documentLibrary/m...er_7360.1_.pdf
Page A21 of this order indicates that "B721" is still an accurate identifies for a stage 3 B727-100, although it appears that the designation "R721" (page A128) is more accurate.
I just attempted to contact Juan Cortes via the email you provided earlier (again, thank you!) but it was bounced back to me as not deliverable as addressed.
I will PM you my email straight away.
Dave