Hi. Can someone help with identification of a Speed Twin. The bike I am trying to buy is believed to be a 1951 Speed twin. The problem is that part of the frame number is covered with paint. The info visible is .... 91 NA ( the NA is correct for a sprung hub). The engine number is 5T 4646. This may be from a later model. Can someone confirm what is going on here? In anticipation, Thanks.