Block version 0x20000000
The bytes you would put in the block header would be, in order: 0x00 0x00 0x00 0x20. That's how the number 221 (= 0x20000000) is stored in 32-bit little endian encoding.
Note that this is actually a large number; that range is used for BIP9-compatible versions. Older blocks that use versions 1 through 4 predate BIP9.