If you want to convert your G-tuned Seagull Merlin M4 to a D-tuned one (using the same string gauges that are factory-shipped), you'll need a different bridge saddle.
And of course, if you're looking to replace a lost or broken one, here you go!
Why do you need a different saddle to tune to D? String thickness. G Seagull Merlins are shipped with lower gauge (thicker) strings than most, so the spacing at the bridge is slightly different. If you try to simply change your G over to a D without a different bridge you'll likely notice times where it sounds out of tune. Potentially at every fret except an open string.