I didn't know what to make of them, so I did what any internet denizen would do. I posted the pics on Reddit and asked people what they thought. Commenters were curious and speculation abound.

I recently went to see them for myself for the very first time, and I must say: they're impressive.

Given the fact that the Millennium Park was once the site of a city dump, many thought the stone tablets came from the Mechanics Hall in Boston, a huge auditorium that once stood where the Prudential now stands.

But, as my colleague Heather Hopp-Bruce reports in her multimedia story "Ten unsolved, creepy, or just weird Boston archaeological discoveries," that origin theory turned out to be wrong.

The stone tablets actually came from the First National Bank of Boston, according to Boston city archaeologist Joe Bagley.

They adorned the outside of a handsome bank building on Federal, Franklin, and Congress streets that was described by the Globe as "one of the largest banking rooms in Boston, and from an architectural standpoint, one of the most distinguished."

An article from an April 1908 issue of the American Wool and Cotton Reporter described the bank's exterior as being made of white Chelmsford granite with large windows facing Franklin Street, "and below them are five marble bas-reliefs, representing some of the chief commercial interests of the bank and city."

The article went on to describe each of those panels in great detail. "The center is Transportation, with a modification of Boston's seal in the center, and the motto of Boston, England ... above. The two panels on either side are respectively Wool and Cotton, and Paper and Lumber; the ornamental features at the top being taken from the coat of arms of the English Boston, once famed for its wool export trade. The end panels are Leather, and Iron Machinery."

Those two end panels -- Leather and Machinery -- were the stone slabs that my mother found embedded in the dirt along the Charles River.

They were an elegant feature on the façade of the building at 70 Federal St., which was home to First National Bank of Boston from 1908 until the 1920s.

After First National moved to a new location, Lee, Higginson & Co., an investment banking firm, purchased the property and moved into the building in 1925.

After Lee, Higginson & Co. vacated the building in 1932, the Globe reported that the building at 70 Federal St. was unoccupied except for a few months each year, when the Greater Boston Community Fund turned the place into their campaign headquarters to raise money for their fundraising drives.

The building was eventually demolished in 1944.

Hub Building Wrecking Co., a company based in Cambridge, was tasked with razing the building.

The vault doors, circular staircases, copper skylights, and other fixtures from the building were put up for sale.

Two bronze doors from the bank entrance on Federal Street were sold to the Malden Trust Co., and some of the granite blocks were trucked to a company in Quincy that planned to turn them into cemetery monuments.

On May 27, 1944, the Boston Evening Globe reported that "a 100-foot steel derrick, mounted on a motor tractor standing on Federal St., lifts the big blocks -- ranging in weight from one to five tons -- out of the structure and gently lowers them to the street surface to be trucked away."

And at least two of the stone tablets that once adorned the side of the grand building were hauled away and dumped in West Roxbury, only to be rediscovered all these years later.

So how did two of the stone panels end up in plain view on the bank of the Charles River?

Personally, I like to think that whoever dumped them at the landfill in West Roxbury -- perhaps an employee or contractor of Hub Building Wrecking Co. -- took a closer look at the elaborate carvings and appreciated the architectural details so much that they opted not to bury them, and instead left them face up for anyone to see.

It's certainly possible, according to Bagley.

"We're not really sure why they ended up exactly where they are," said Bagley. "If that was deliberate, or if that was just, you know, where they stopped rolling."

Now that we know they were among the five stone panels that once adorned the exterior of the First National Bank building, there's also a new question to answer.

"We solved one mystery," said Bagley, "but there's still this remaining one, which is: where's the other three?"

Alas, that's a mystery that won't be solved anytime soon. As it stands, Bagley said the city has no immediate plans to try to locate those other three stone tablets.

"Wherever they are, they are stable and unthreatened, which is ideal for archaeological materials," Bagley said.