The key appears in different degrees. This particular one is from the fourth degree (“Secret Master”) of the Ancient and Accepted Scottish Rite.
In Emulation a single key is the emblem of the Treasurer.
A key also appears on some tracing boards of the Ark Mariner degree.
Maçonnerie des Hommes (1766, Vol. V. degree “Knight of the 12 Tribes) says about the key:
The key, that you can also see on the tracing board, is a symbol that teaches you to cherish the secrets that are given to you in your heart, it is like a sacred treasure, which the Enlightened Brethern entrusted you.
The Book Of The Ancient And Accepted Scottish Rite (1884) Charles McClenachan