Looks like decals were likely added sometime after it left the factory except maybe the seat tube that says Reco. There is patina (oxidation) where the paint was scratched up. There would be need to sand down the patina if you plan to refinish it. The paint may be original or was painted a long time ago. Will need refinish and replacement decals if you want museum piece.
If you like the rat rod style - it may work as is. 1977 serial frames with lower gusset are hard to find. Please contact me if you have any questions.The item "Old School Bmx 1977 Mongoose Frame Lower Gusset Patina" is in sale since Monday, February 15, 2021. This item is in the category "Sporting Goods\Cycling\Vintage Cycling\Vintage Bicycle Parts". The seller is "dcbmx1" and is located in Fremont, California. This item can be shipped to United States, Canada, United Kingdom, Denmark, Romania, Slovakia, Bulgaria, Czech republic, Finland, Hungary, Latvia, Lithuania, Malta, Estonia, Australia, Greece, Portugal, Cyprus, Slovenia, Japan, China, Sweden, South Korea, Indonesia, Taiwan, South africa, Thailand, Belgium, France, Hong Kong, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Bahamas, Mexico, New Zealand, Singapore, Switzerland, Norway, Saudi arabia, United arab emirates, Qatar, Kuwait, Croatia, Malaysia, Brazil, Colombia, Barbados, Bangladesh, Bermuda, Brunei darussalam, Bolivia, Ecuador, Egypt, French guiana, Guernsey, Gibraltar, Guadeloupe, Iceland, Jersey, Jordan, Cambodia, Cayman islands, Liechtenstein, Sri lanka, Luxembourg, Monaco, Macao, Martinique, Maldives, Nicaragua, Oman, Peru, Pakistan, Paraguay, Reunion, Viet nam, Uruguay.