Mineral property still owned

I have mineral rights in Richland County, Montana. In 2005 I owned 11.85185 net mineral acres on a 320-acre lot. Now in June of 2005 I sold 6 of those acres and was sent a check from McCartney Petroleum Inc based out of Sidney, Montana. I acknowledge I did get one check from them and did indeed sell that 6 acres, but they are saying now and in fact sent me their deed paperwork that I did sign selling my remaining 5.85185 acres. Now here is the part that is confusing: I did get the first check but the second check I never got. I moved. I know with the first one it came certified mail and they are supposed to keep records and well as I should have. I have contacted Mr. McCartney who is now retired and sold his company to another oil company and explained the situation. He looked through his records and only had a tracking number for one of the checks. Yes, he said he sent out both of them but I only received one. My question is how do you go back to 2005 and find this info out to verify that indeed you did not receive the second check due to my moving? I believe I never signed for it and therefore shouldn’t I still own the mineral rights to that Section Township 22N Range 59E Section 27 W1/2? I am going to contact a lawyer tomorrow because I know I never signed or got the second check. Thank you for any input. Theresa Hennen.