Old double-glazed window

One of the original windows in my house (53 years old) is a nice side-to-side slider with double-glazed sashes. One of the outside panes of glass is cracked and I'd like to replace it.
Here's the problem though...
The outside pane of glass has a wood frame and is screwed to the inside frame. Until now I've never noticed any condensation between the panes of glass, but I wonder if I replace this pane if I'll inadvertently allow moisture or other undesirables between the panes.
I don't think they were using vacuum machines on these windows 50+ years ago. Also, I'm certain there is no gas between the glass panes. And the screws fastening this thing is are big and certainly look like this was supposed to be maintainable.
So how did they (and how can I) assemble this so as not to have a problem with condensation between the panes?
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