I just saw your post by chance and wanted to throw in my 2 cents.
A sharp chisel or a rigid putty knife and hammer are one way to get that old putty out. These tools need to be sharp and place the bevel side of the tool against the wood so as you hammer you wedge off the putty.
*Note that most glazing compounds pre 1970 have lead in them so I would discourage using power tools that will create a lot of dust. Use a spray bottle with H2o in it to keep the dust down and Vacum using a Hepa filter vacum.If hammer and chisel are useless,,, too slow or if putty is still soft ie:not coming off nicley in chunks.
Use a heat gun and the putty chisel. Heat the putty slowly and evenly. Moving the gun side to side. Heat an area 6-9 inches at a time. The paint should start to change on the putty. That is your cue. Then wearing leather gloves scrape the putty off with the chisel. Always scrape parallel to the glass. Heat the next area and repeat.
I have been a stained glass worker for the past 12 years. I have removed a few miles of putty without breaking much glass. I have used a fein tool, Ed Huntress has the right technique.
JHoaglund