There are several possibilities,
Generally speaking the skull and bones (often refered to as Jolly Roger) has a strong connection to pirates, hence why it has sometimes been used by naval troops throughout history. This however would not account for the infantry association in your helmet, unless of course your helmet was reissued before the application of the insignia, which is very much possible.
The engineer/bomb disposal unit suggestion is a really good possibility as well. There is a very good chance of that and probably is the most likely of all possibilities. The skull and bones insignia most often signifies death or danger, which of course is why it has been the most appropriate to be used on warning flags and such. Considering this, perhaps there's also a chance of this helmet having belonged to an army coroner or someone in charge of removing fallen soldiers from the battlefield? Perhaps the latter sounds a bit too disrespectful, but I can imagine the people who had to do this work liked to lighten the mood with this kind of thing.
Thirdly, there is a possibility of it being a personalization, USAAF bomber crews would sometimes paint whatever was painted on their aircraft on their helmets as well. The same may have been done in certain infantry units, armored infantry units in particular spring to mind.
Finally, this one is pretty far fetched, but the Germans had an SS Totenkopf division that used the skull and bones as well. Unlikely as it may seem, there are pictures of German troops wearing allied helmets (see picture attached). This is thinking out of the box, far out there and ridiculous thinking perhaps, but every now and then you will come across period pictures that go to show there is no real 'normal' when it comes to collecting helmets. Frankly I'd rule out this possibility, it is just very unlikely but the pride some Germans took in wearing allied steel still is an interesting historical fact and what better time to share this than now?
The insignia in any case looks original, I hope you can figure it out. Good luck!
Edited by ken88, 03 December 2019 - 02:24 AM.