So, I bought a Hamilton automatic from a dealer yesterday.
He had attempted to open the case to check the hall marks prior to buying it himself but discovered it was a front loader. He then tested the case back and filed and tested an area inside the lug and both tests showed the case to be gold.
There are no signs of wear around any of the edges of the watch so no visual tell tale signs that it isn't solid gold.
My reservation is the Hamilton stamp "10k RGP" on the case back.
Normally to me I would take that as meaning 10k rolled gold plated. I have being doing some research on Hamilton cases which albeit not definitive does point to it being more likely plated. What I don't understand though is the acid test results which were done by someone used to buying and testing precious metals.
Has anyone any thoughts or experience with these type of marks / cases? Neither myself or the dealer are able to open the case but before going back querying it any advice would be welcome please. 
