Values of fields OrderID, TradeID, MDEntryID are encoded from UInt64 to 12-symbol string.
First char in encoded ID is prefix. Possible prefix values are:
'O' - Order,
'T' - On Book Trade,
'N' - Off Book Trade.
The rest 11 chars represent original ID in Base64 encoding (similar to Base64, but without '=' suffix and based on 62 characters set (10 chars ‘0’-‘9’ + 26 chars ‘A’-‘Z’ + 26 chars ‘a’-‘z’).