In a room behind the teller line at the Edgartown National Bank is a door into a 10-by-6-foot structure. It looks like an ordinary wood frame door, with molding and an antique doorknob, until you try to open it. Careful, it’s easy to fall over when opening the 560-pound door to the old safe that dates back to the 1850s.

This weekend, the safe will be removed from the bank’s South Water street location to make room for more office space. Fielding Moore, bank president and chief executive officer, offered a glimpse of the historical gem before welder Daniel Serusa takes it apart. Mr. Serusa marveled at the construction.

“They built that safe in a factory off-Island, and the safe was shipped here and somehow they got that thing up onto the top deck and built the bank around it,” he said. “Back then, for them to fasten steel . . . they didn’t have welding, and everything was riveted together. To get this thing apart all rivets have to be cut.”

The 1855 brick building was originally the Martha’s Vineyard National Bank; a Gazette story on Sept. 23, 1955, reported the bank sold the building to the Edgartown National Bank for $2,500. The national bank was chartered on Sept. 2, 1905.

By the time the Edgartown bank moved into the new space, technology was such that concrete vaults were poured into place. Since then, the old vault has sat unused.

“We want to use the space and looked into how we remove it. We knew there was brick around the outside that encased it and had to come off,” Mr. Moore said. “What we have is a steel cage. We didn’t expect to find modern-type steel there; we thought it’d be fairly easy to demolish given 1850s technology. I didn’t realize modern steel mass-produced was patented in 1851.

“We didn’t know what it was, but when we got the brick off we figured it out.”

Mr. Serusa and three helpers will begin removing the safe tonight, and hope to have the job completed by Monday. The biggest challenge will be maneuvering the doors out of the bank, for which he built a custom-sized dolly. Mr. Serusa has a few antique dealers interested in buying the doors.

With six-inch thick steel plates, the combination lock is still on the back of two residential-sized doors, totaling 1,440 pounds of steel. But the greatest historical reference on the safe is barely noticeable in the dimly-lit construction site in between the tellers and the ATM machine. On a side plate that separates the two doors to the safe is a signature: Dr. Daniel Fisher, Edgartown, Mass.

“We’re assuming that Dr. Fisher, the whale oil baron here in Edgartown . . . was instrumental in buying the vault that was put into the bank,” Mr. Moore said. “He probably needed a secure place to put all of his money.”