Riki and Niki Buchanan of Diamond Estate Services tell TMZ ... some of Clint's OG treasures from his former home in L.A.'s Beverly Glen neighborhood will be going up for sale -- including original cowboy boots and belt buckles from his western films.

That's not all ... there are also framed movie set photos, vintage movie posters, director's chairs, a massive vinyl collection and even his original gym equipment and arcade games.

Translation: it's a true Hollywood jackpot.

Clint clearly amassed quite the collection during his 20 years in that home -- it was his first in L.A. after he'd snagged his first big TV role in "Rawhide."

In 1983, he sold the home to his close friend and Eastwood Films producer, Fritz Manes for $250K. Fritz lived there until he died in 2011, and his wife remained in the home until she died a few months ago -- hence, the estate sale.

The couple kept the place exactly as Clint had it, leaving all his memorabilia and furnishings untouched.

Now, the Manes estate has enlisted Diamond Estates to host a one-of-a-kind sale. As for pricing, all items are currently undergoing appraisal.