A concept car from the height of the muscle car era has reappeared after decades in hiding, and it's headed to auction.

The 1970 Plymouth Barracuda Rapid Transit is one of four show cars built for the Rapid Transit System Caravan, which toured auto shows and car dealerships in 1970 and 1971 promoting Plymouth's muscle cars. Stored for over 50 years, it's scheduled to be auctioned off by Mecum in Indianapolis in May.

Launched as part of Plymouth's period Rapid Transit System ad campaign, the Rapid Transit System Caravan included a Duster, Road Runner, drag-racing star Don Prudhomme's Plymouth funny car, and this Barracuda, which was customized for the tour.

A paint scheme designed by Hot Wheels artist Harry Bradley is laid over bodywork modified by Chuck Miller, a Detroit-based custom car builder who won the prestigious Rider Award in 1968, and had built a life-size version of the then-famous Red Baron hot rod model kit before being brought in to customize the Plymouth.

Working in steel, Miller made a new nose from four identical angled pieces welded into a single insert, per the auction listing, as well as a rolled front pan with split spoilers and low-mounted driving lights. Side pipes, steel mud flaps, a fake parachute, custom taillights, and wheelie bars were also added, along with wider tires and Cragar wheels.

An early-build example, the Barracuda has a 440-cubic inch V-8 believed to be the engine it left the factory with. This more powerful engine, which inhales through a Shaker hood scoop, would have rated 'Cuda nomenclature back in the day. Plymouth began using the 'Cuda name to denote performance versions of the Barracuda when this generation was introduced in 1969.

Following the conclusion of its promotional duties, the customized 'Cuda fell off the radar. According to YouTube channel Auto Archaeology, the car went into storage in 1976, and remained hidden until earlier this year. It's in good condition for such a relic, showing just 967 miles on the odometer and still wearing its original paint, according to the auction listing.

Barracudas are some of the most desirable muscle cars today. Another 1970 example claimed to be the first Hemi Barracuda was offered for sale earlier this year with a $2.2 million asking price. So this well-known one-off could bring serious money when it crosses the auction block.