I'm not clear on the oval..... is it an area that was left unpainted, with green painted around it? Or was the tan oval applied over the green paint? Either way, although I don't normally opine on the authenticity of helmets that I have not personally examined, I remember seeing that helmet on eBay and not having a great feeling about it. I think it had most to do with the backwards eagle, but other factors included the large tan oval as well as the design of the stencil. But as I have said here many times, that there are exceptions, is often the rule.
Here are a few photos that have some relevance to the discussion. The first is a French 1927 Adrian that is painted green, but which was originally painted a khaki color. The triangle appears to have been masked off when the helmet was painted green, leaving the triangle which was then outlined in white. It's hard to see in this photo, but the helmet also has a light blue band around the base. I am unsure as to the significance of this triangle/band/eagle arrangement (a specific purpose or a specific unit ?? Airfield guard has been suggested).
The second helmet is a 2 Corps MkII with camouflage applied around a white hand-painted eagle, which was on the helmet prior to the camouflage application. So, seeing the application of paint around an existing eagle is not unheard of.