The HGU-40/P helmet appears to be an upgraded version of the MB-3 helmet. Upgrades include a new AN/AIC-10 compatible headset, a formfit liner and lightweight bayonet receivers.
The designation was assigned in 1975, and evidence from a former B-52 tail gunner stationed at Ellsworth AFB shows that the helmet was distributed for a six-month operational testing in 1978. This particular test individual much preferred his standard issue HGU-26/P.
The HGU-40/P was also evaluated in fighters as a sidebar during the Aimval/Aceval AMRAAM tests at Nellis in the late 1970s when a lightweight fighter helmet was sought. The winner of that helmet evaluation was the HGU-48/P, the forebear of the HGU-55/P.

HGU-40/P helmet with MBU-5/P oxygen mask. Click on thumbnails to see larger picture.
Pictures © Howard Jones, Spanky and eBay seller csardude