Notice how the steel muntins and bars are lapped into each other and tenoned into the frame!

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In our analysis, we were  surprised to learn that the strength and stability of the steel sash were based on the tenons and lap joints and not the  expected to be welded joints. This is  understandable from a historic  evolutionary standpoint. Steel windows were a replacement of wood using new industrial methods and materials. The steel men copied the fabrication and assembly  methods of traditional carpentry joinery.

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Conclusion: These steel windows are unique, salvageable and historically important. Secondary strategies to improve thermal performance would be best performed with an easy detachable internal energy panel.