Hi friends, looking for any advice on this.

This recess (about an inch deep) is in the top of my flat entrance door near the hinge side. It appears to be a red plastic casing that's lining it, the pink bits are just paint. Unfortunately I cant get a better picture because of where it's situated. I've searched high and low and can't find anything out about it. This door is 21 years old and I am convinced it's solid wood (chipboard or similar engineered not solid timber). I'm basically looking for any evidence that could demonstrate this to a high degree of certainty as I cannot get inside it.

Is there any chance at all that this could be/could be the remnants of, an older style plug? Or is it just one of those random manufacturer things that means nothing at all?

As i say, proving it's solid composite wood - and not composite as it's typically understood i.e with some kind of thermoplastic outer/pir core - is my priority, anything else is a bonus.

Any ideas on how else i might prove this based on what you can see or indeed, confirmation that i am wrong and it is the other composite, also welcomed!

Many thanks in advance.

I'd assume it's a manufacturer plug as it doesn't match either old roundel or newer tree type plugs in main colour.