Hello RTTY,
There is a lot going on here, so I will break it down into bits.
Firstly, it is not rare. A coat of arms in the UK is granted to one individual at a time. So these arms are no more rare than my own.
Where was the picture taken? That might give a clue to who owned the arms.
I agree with sun and crescent and eagle displayed.
It is not a plant, it is mantling. A short, curtain-like affair that extended from the top of the helmet to just below the shoulders. It was designed to keep the sun off the helmet and shoulders. Many of the original crusaders died by cooking inside their armour. The mantling should not be confused with a mantle (cloak). The cloak fastened at the chest (not neck) and performed the same function as the mantling. Originally, both were white on both sides. It looks the way it does because it is trying to look as if it has been torn and slayed in the heat of battle.
It is a "tilting helm" used for jousting. The rider would tilt his head forward, so he could see out of the eye slits. Just before impact, he tilts his head back to stop the opponent's lance blinding, or killing him.
The crown is called an "ancient crown" and has no special significance. It forms part of what is called the crest. That is to say, the crown and the eagle are part of the same thing.
The eagle, is single headed. It is holding a snake in its beak. The snake's head is to the left, and the body forms the number 6, curls back on itself and the tail looks like an up tick.
The motto "Proxima soli" - this is something of a mystery. I suspect a mistranslation somewhere along the line. Soli when put with Proxima is implying some proximity to the sun. Whereas "Soli" on its own, translates as soil or earth. Possibly giving a transliteration of "Stay Grounded".
All Masonic tombs that I have seen, display the square, the compasses, the all seeing eye, a chequerboard, or a sun in splendour - or some combination of one or more of these elements. There is nothing in this representation that would indicate masonic to me.
If it were me, I would start with the nearest town or village to the graveyard. Find out who were the prominent people living there and see if there had been any endowments to the local church. If a man is going to have an impressive stone, he usually gets buried in his local churchyard rather than miles away.
Regards
Chas
