The below info is just for reference if interested of how the Mercury outboards serial numbers start out as,
(U) = UNITED STATES: Starts With Serial Numbers 1, 2, 3, 4, 5, 6, 0A, 0B, etc..
(A) = AUSTRALIA: Starts with 8
(B) = BELGIUM / Europe: Starts with 9
(C) = CANADA Starts with 7
(I) = INTERNATIONAL Starts with: 0N
While it could be possible the info you have is correct about it being an 1984 25-HP-XD, The serial number doesn't seem to add up as being an XD engine model but rather a normal 25-HP engine according to the parts catalog serial number range sequence.
Your number being 9400389 suggests that it falls more within the non XD 25-HP range which is
25 - 9378116 THRU 9415100 VS
25XD - 9415101 THRU 9507380
http://www.iboats.com/MERCURY-25XD-1984-1987/dm/view_id.1701517
http://www.iboats.com/MERCURY-25-HP-2-CYL-1980-1983/dm/view_id.1701474
That mentioned, this same type of general question/info seems familiar and if I recall correctly there/it was an 1984 25-XD engine but it's not showing up in any of the usual website charts listings for an exact match....So to play it safe, you may have to use both the serial number ranges above and compare notes for the parts you need until you know for sure.
Any engine from Belgium will not be recognized by a computer data search at least here in the US.
What other type of info where you looking to get if you're sure of the engine model and year? A model number will not be possible to look up for this serial number.