I've found a search engine result for an OEMMAX MCK-860 which suggests that yours is (also) Ortek. If so, then we finally know what MCK stands for in Ortek model numbers: Multi-Compatible Keyboard. Which then raises the question: does that mean that MKB keyboards are non-switchable? For example you have MCK-84 and MKB-84, and the difference between them is not known.