This location is at the Scottish Rite Temple in McAlester, OK. While a Masonic organization and all members must belong to the Masonic Fraternity, it comprised members who have chosen to attend Consistory, which is to "go to Masonic University," so to speak. The highest a Mason can go on their own merit is the 32nd Degree. All degrees, or recognition, after that are honorary.
While most are familiar with the local Lodge concept, Scottish Rite bodies are comprised of men from many different lodges and so each temple is designated as a Valley. There are 3 in Oklahoma, McAlester, Tulsa and Guthrie.
The Scottish Rite has it's own philanthropic work, the most prominent being Rite Care.
The Valley of McAlester was even the home of one of the most famous Oklahoma sons...Will Rogers. It may be possible for you to inquire about a tour of the inside which houses an accoustically perfect auditorium, which also has a lighting system that is still partially operational and is listed on the register of National Historic Places.
Twice a year there are reunions held which brings together AASR (Ancient and Accepted Scottish Rite) Masons from all over and may make seeking the cache quite difficult. Other than that, it should be an "easy" task and they are aware that this one is on the grounds.