British Mk4D Aircrew Flying Helmet
For Night Vision Goggle (NVG) use, a special inset visor was manufactured by Gentex USA, based ona a silimar concept for the USAF HGU-55 helmet.
This visor offered wind blast protection, and allowed for the fitting of NVG's to a bracket in the brow area of the helmet - conventional visors can't be lowered when NVG's are in use.
The pivot of a Mk4A helmet is used, the pop-studs usually used for the visor cover are used to mount the inset visor with elastic straps.
The Mk4D was also fitted with a LEMO multi-pin conenctor instead of standard NATO plug, which provided power to the NVG mount on the brow of the helmet.
These visors in use with a P/Q mask provide a similar NVG soution to the X-Type mask used with a Mk4B/4L (or Mk4A4, stil to establish which), however, I'm yet to discover which superceded the other