What's Up With This Mark?

[The Wizard]

I was cleaning my Henry Nettleton US Army service revolver after a day of poluting a hill side with lead, when I noticed for the first time a "T inside a D" mark on the forward, right side web of the trigger guard.  Anyone know what the mark is for?

[mtone]

Is this an earlier production HN or later one?

[TDW]

I believe that's the house stamp for Doug Turnbull.
Tom