I once had a "lawsuit" Takamine to which I attached a rosewood armrest. I think it was on the instrument when I dealt it. Fwiw, I don't have trouble keeping my forearm off the top or the edge from bothering my arm so it doesn't seem necessary to me now.
If you do get one, match the radius to your guitar closely, otherwise you get a ridge which just digs into your arm.
I have two of them; one on my Bourgeois JOMC and one on my Thomsley OM. I will probably add one to my Bourgeois DBJC, as well.
They do offer some benefit in the tone department by not allowing the pressure of your forearm to dampen the upper half of the lower bout. This is more of a benefit on smaller body guitars or instruments with very lively tops.
It is easy to remove if you find you don't like it. I have changed them around on a variety of guitars with no ill effects.
Twelfth Fret, and many of the larger shops keep the basic model in stock. I ordered mine on-line from FQMS.
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