2004 should be a drop in. I think Martin swapped to drop-in in 2003.
If the saddle ends look like this, you can not put an undersaddle transducer and should try a soundboard transducer (K&K, ultratonic, Dazzo, HFN…)
If the saddle ends look rounds it is a drop in
In this case, the saddle slot is routed through and you can put an undersaddle transducer like Fishman Matrix or Baggs element (used in Anthem). The router bit is round and so are the saddle slot ends.
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Martin 00-18V Goldplus + internal mic (2003)
Martin OM-28V + HFN + internal mic (1999)
Eastman E6OM (2019) Trance Audio Amulet
Yamaha FGX-412 (1998)
Gibson Les Paul Standard 1958 Reissue (2013)
Fender Stratocaster American Vintage 1954 (2014)
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