You could all always take the easy route and have an AB stem made for a 309, I know it's not the same as having an 'actual' 4AB, but it might be quicker and cheaper. Funnily enough, we have some great stem-makers on the forum, naming no names...
I concur completely - I did not read your post prior to posting above. I have a 1905 AB stem that fits a few of my older 309's, and it's much closer to what Rathbone smoked.