These are rather an interesting helmet and are quite hard to source there is nothing extraordinary about the helmet body it self it is a post Vietnam era produced example the real prize hear is the liner it is a modified type II jump liner.
Take special note of the A-yokes these have been constructed with slide buckles instead of the standard brass wire claw buckle system for attaching the chin cup which proved to be unsatisfactory in Canadian trials as they were found hard to adjust and the eyelets in the chin cups had a tendency to to tear away.
This modification is thought to have been instituted and adopted post January of 1981 I did not recognise the dome stamp in this example when I first received it and had to do a little searching around but the liner webbing was nicely marked ( Liner Parachutist Helmet DLA 100 81 C3597 Speciality Plastic Products inc..)
If I am reading the DLA number correctly it puts this liner as produced some time in 1981.
The extra added bonus is the rear shock absorption pad which sole purpose was to be used during the jump for extra support is still present as on the norm they are missing.