What is going on here?

- /sdef/Aircrafts/Engines/PackardMerlinProp folder has replacement sdefs for the built-in P51 sounds.  These are necessary to suppress the stock sounds, do not alter the SDEFs.  They supress by pointing to a nonexistent .wav.
- /sdef/Aircrafts/Engines/LancasterMerlin folder has SDEFs used by our new custom sounder.  The SDEFs are called from Sounders/Aircraft/Engines/Engine.lua
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