It's been billed as an attempt to solve one of Rock and Roll's great mysteries.

A global search is underway to find a bass guitar that once belonged to Paul McCartney and featured in several iconic early Beatles songs including "Twist And Shout," "Love Me Do" and "She Loves You."

The guitar, a Höfner 500/1 electric bass, was purchased by McCartney for around $37 in 1961 from a shop in Hamburg and said to be his "favorite," according to The Lost Bass Project, the group which has launched the new search.

"This is the bass McCartney played ... in Hamburg in 1961, at the Cavern in Liverpool and on those first Abbey Road recordings -- it powered Beatlemania and shaped the sound of the modern world," The Lost Bass Project said.

The group, a team of dedicated Beatles fans and researchers, noted that McCartney had used it in London in 1969 when the Beatles were recording the Get Back/Let It Be project -- and had not been seen since.

"Nobody has ever clearly established where the bass was stored ... and nobody has come forward with an account of what happened to it," the group said, adding that its disappearance fueled rumors, conspiracy theories and false sightings over "the last 55 years."

"The murky world of stolen guitars is now awash with forgeries and fakes. All sorts of rumors have emerged about where the bass went and where it could be," the group said.

Launching the project on Sunday, the group said they were "carrying out targeted research based on existing information and insights" and were now in the midst of "gathering and responding to new information and insights shared by people around the world."

"Paul McCartney has given us so much over the last 62 years. (This) project is our chance to give something back. If you have credible information about the bass, this is your chance to share it and be part of music history," it added.

'Most important bass in history'

The project's public appeal was launched on September 3 and hundreds of new leads have already poured in, the group says.

Heading the Höfner search project is Nick Wass, who worked closely with McCartney's team and even wrote a book about the missing instrument.

"It was very likely stolen," Wass said in a statement shared on the project's website. "Someone somewhere knows what happened to this bass and where it is now."

"Paul would be so happy, thrilled, if this bass could get back to him," he added. "I know because I talked with him about it."

But the team of music sleuths is hopeful.

"Lost guitars can be found," it said, specifically citing a Gibson J-160E acoustic which once belonged to John Lennon and disappeared during a performance in 1963, only to resurface 51 years later.

"It was also very distinctive, no other 500/1 bass looked like it," the company said.

To aid in the search, the company has provided a list of specs to look out for if you think you've come across McCartney's long lost instrument, which include a solid spruce top and 3-piece neck as well as "two Diamond logo pickups and no poles."

Another "dead giveaway" is the Höfner logo, the group says, "written vertically on the headstock of the original model but horizontal on later versions played by McCartney."

But it could end up looking very different, given that it's been missing for decades.

"The bass was in need of repairs and so it was sent to a firm in London, early in 1964, who carried these out. They resprayed the bass a darker three-part sunburst and fitted new knobs," the company added.