You need to retain the same base name for the aircraft m3d (Xxxxxxxx.m3d), the cockpit m3d (Xxxxxxxx_cockpit0.m3d - following cockpits if applicable are numbered sequentially, so Xxxxxxxx_cockpit1.m3d, Xxxxxxxx_cockpit2.m3d, etc) the air file (Xxxxxxxx.air) and the xdp file (Xxxxxxxx.xdp).
You also need to open the xdp file and correct the entry here:
<?xml version="1.0"?>
<UnitData>
<General Allegience="0" LongName="3GB Beaufighter Mk 1" ShortName="3GB Beaufighter 1" ModelName="3gb_Beaufighter_Mk1.m3d" BlastDamageScale="1.0" ImpactDamageScale="1.0" FireDamageScale="1.0" Priority="2" Type="moving" EnteredService="09/01/1940" LeftService="4/1/1943" BlastDamageMod="1" ImpactDamageMod="1" FireDamageMod="1" Category="fighter_bomber" Country="britain" AllowSpawn="y" InfoURL="help\topics\infomosq.htm" ChecklistURL="help\topics\infomosq.htm"/>
So that it reflects the correct name. Use Notepad.
Finally you need to change the entry in the aircraft.cfg file:
[fltsim.0]
title = 3GB Beaufighter Mk 1
sim = 3gb_Beaufighter_Mk1
model = 3gb_Beaufighter_Mk1
The sim name and model name are the same as the m3d name without the tag. The title line is redundant.
Hope this helps.