Just as Rubie's FG LE is the more prestigiuos fiberglass cousin of the lower-priced plastic Rubie's Deluxe, the Don Post Deluxe was the higher end fiberglass couson of the lower-priced plastic Don Post Classic Action.
However, it is clear that the two plastic helmets are non-identical.
Could they, however, be related in some way? It is possible but difficult to establish.
Other than the obvious, one fundamental and foundational flaw of the Rubie's is that the eyes are pressed together too closely at the sides (the temples). Purists with access to accurate prop replicas will notice this very quickly.
The DP CA suffers from the same problem. When looking downwards at it, observe the shape of the lower eyelids. The DP DLX's eyelids have a deep "U"-shaped bowl-like profile, whereas the Don Post Classic Action's "U" actually starts to narrow to begin to resemble a "V". However, the DPCA compensates for this by having a narrower skull profile so that things do not look utterly out of proportion.
The Rubie's however is a different story. If the mold for the DPCA ended up in Rubie's hands and the master was modified where the rear opening was pulled wider to make the skull bigger, the eye and area will still appear pressed in at the sides. This is extremely difficult to correct structurally on the Rubie's. Typical fan mod tutorals may improve on the appearance of the Rubie's but do not sufficiently address foundational probelms like this.