Not a proper Roman numeral. D is 500. X is ten It could be 510. Or it could be in improper attempt to write 490 (which would be CDXC).
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It is: XXI = 21
The number 1776 in Romannumerals would be MDCCLXXVI
XD = 490
Yes and XD would mean -10+500 = 490
Under today's rules 490 expressed in Roman numerals is CDXC but the Romans themselves would have probably preferred XD.