I hadn't thought about doing it from the outside, I'll have to look into that one. Yes, I see a lot of 1 1/2" flat stock with the trim nailed over half of it. You'd think they would have made those windows so they could easily install replacement vinyl windows, like they did for us at the turn of the century

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I never find any directions on how to install these windows in that type of window, everything is geared toward the older windows with stops.
I also never see much on just replaceing the side jambs, don't know if they are any good.
I'd take pictures but I'm not currently privy to these types of windows. My house has the old beveled leaded glass with stops, and so I put storm windows on.