For anyone interested, I checked the Glenwood info that Chad ( vfw3439) so generously posted here (thank you, once again Chad!). The Holy Grail of Glenwood Heater Documents
The Glenwood Oak and the Glenwood Hickory were available with the optional back pipe (indirect stoves). In the pictures it looks like the same arrangement as the back pipe on the Glenwood Modern Oaks.
If anyone has a Glenwood Oak, or a hickory , especially if it has the cast iron base support (cup) attached to the stove, and if it uses 6 inch stove pipe, these cast parts from the Modern Oak 118 back pipe project, can likely be used to make up a back pipe for those stoves too, while giving the look of the originals.
Also, . . since 6 inch stove pipe is such a common size for many of these Oaks, it's may be possible to adapt them to non-Glenwood Oaks. Maybe someone can check theirs and post what they find ?