This is not a perfect post, both the Dragons' Teeth and the third sub-piracy are absent, but by the end you will at least be able to pair-up most of your Giant and sub-Giant bunkers and barbed-wire, if you currently have them all in one tub/box/place!
But we start with the granddaddy which is Marx, specifically the Miniature Masterpieces accessory which was, itself, probably pantographed-down from the larger item found in the 54 and 60mm play sets.
And odd design; more knights helm than pill-box, but having something in common with some of the cheap iron or concrete 'sniper' posts used by various armies/nations since the end of the 19th Century, except they are usually small one- or two-man things.
Hard polystyrene plastic and marked clearly with the full stamp, this one 'Hong Kong' but there may be 'Taiwan' versions out there?
Giant got a copy of the Marx unit out for their WWII sets, it's soft polyethylene plastic, a little smaller, a little less sharply finished, and there was barbed-wire (illustrated) and dragons' teeth (see below) to build a formidable defence-line from multiple sets.A reminder that the that the P-in-a-circle mark; ⓟ, is internationally recognised as the symbol for phonographic recordings, not 'patent', 'protected', 'perfect' or even 'plastic'!
These would seem to be sub-piracies, and I have titled them C1 for 'copy, type one' (for the purpose of this post, you can title them whatever you like!) although it may be that the next lot ('C2') came first, but these are closer to the Giant version, just a poorer copy, less well-finished.The arrows are pointing to large pin-release marks, which are more-like channels, filled with resin which has then been cut away with tin-snips or side-cutters, leaving still quite-pronounced protrusions.
These (C2) are poorer again, but differences in the overall design suggest parallel copying rather than liner, so they may predate the C1, but it will only be by a few months as the sets carrying them both are pretty contemporary - approximately 1966-68.Also - or again; soft polyethylene plastic like the C1's and Giant's original copy.
The barbed-wire which accompanies the Marx-copy/Giant-clones; they are both copies of the giant version - which we looked at here - and a much finer piece than all the clones. It should be noted that while I am 100% on the tie-in between the C1's, with the C2 I have such a small sample of wire it cannot - yet - be taken as empirical, although checking with the Beach Head sets will sort them all out, more accurately, another day. The four together, you can see that with the C2, there is a different profile to the soil the pill-box is set into, and while the surface is smoother (suggesting another pantographing) it is also a cleaner bunker sculpt than the lumpy C1, so one suspects that the C2 is a separate copy, independent of a evolution-line of cloning of the Marx/Giant donors.There is another bunker design, which could be C3, or, a C2 if the brown ones are a stand-alone line/design, as it seems to be copied from the Marx-Giant-C1 evolutionary line, but the 'mouth' has been dropped and a 'side-door' arrangement added to the [defenders] left side (right side as we're looking at them here), they can be seen in the above links, but I can't find them, despite last shooting it in 2015, so they must be here somewhere!
While the additional strand (at the bottom of this shot) may go with the missing 'doorway' (C3) bunker, something else I'll have to check against those sealed sets! Note: double barbs on C1 and smaller barbs on C2 with the question-mark being closer to the Marx original but with four spans against the three of everyone else's.
Basically they all follow the Airfix pattern of a 'hand' of five, linked to make a double row 2-3 ( ,',', ) and I think the Giant are the taller, with the clones getting smaller ones, which - like the barbed-wire - will probably be found to run to four obvious variants? However, all the clones are without the pyramid on the cap, which the Airfix ones wear.
In addition to the links above, this post covered the other examples in my 'master collection', and both posts linked-to owe a debt to James Opie, from who's collection several of them came.
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But is it Giant?
I set a rod for my own back with this footer-feature didn't I!
Yes, if it says so in the text, no if it doesn't and almost certainly no if it's product added to a generic set with a bunch of non-Giant product also present! Another way . . . everything in the second image is Giant, except when it's been supplied to someone else; everything else isn't Giant except where the Giant stuff is being compared with it - in images 6 and 7! There might be some Giant in the last image!
It'll be clearer when we return to them to clear-up the remaining question-marks.