It's a 4mm, 4mm glazed unit with 16mm spacer, built by UltraGlass for $110 (512mmx1250mm in size).
- I removed the existing window frame
- removed the three panes and two muntins
- routed the inside space to allow the much thicker double glazed unit (24mm) to fit
- painted window frame
- used wood beading to fit window in place
- reattached window.