This is the 550b equalizer for the API (エーピーアイ) 500 series. I am selling it to organize my equipment. This unit is a very rare vintage early model of the 550b, which is synonymous with API, with a serial number in the 2054 range. The musical and excellent sound of this period's units has been loved by many professional engineers. It is equipped with two vintage white genuine API 2520 op-amps. Product Details Brand: API (エーピーアイ) Model: 550b Equalizer (500 series module) Features: Vintage, early model Condition Working and in good condition. ※ Although the operation has been confirmed, please refrain from purchasing if you are nervous about used items or if you are looking for perfection, to prevent any problems. I am sorry to ask, but to avoid any unpleasant feelings for both of us, please be sure to read the profile and consider purchasing only if you are satisfied.