I believe this is the head code: 3836850
The head code is for a 261 produced between 54 thru 62. The date code, K21 8, translates into November 21, 1958.
From what I been reading, Flxible Clippers never came with Chevy engines. They used a Buick straight eight and a GM diesel option, so the number BGD 688104 is more than likely not Chevy. You may have better luck finding out what engine came with your bus on a Flxible site.
