Movement ring for sapphire case back on Speedmaster...do i need this thing?

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In this case, it's an integral part of the case and performs multiple functions. Even though an 1861 movement is less susceptible to magnetism, it can still be affected, and the sof iron dials and caps are more permeable to magnetic fields, so they help direct the field away from the movement itself. It's not perfect, that movement can still be affected by magnetism, but it won't stop, which is the major problem. Even the 15,000 gauss watches can be affected by magnetism, but they will still keep time within spec. For example, when the 1957 Railmaster I had would get magnetized, it would run about +3 to +4, and demagnetized, +1 or less. Still within spec, just not as good.
 
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Either way the cap is not fitted on the display caseback models so it can't be that important in dust or mag prevention.
Right. You're not going to space with it, so it's less important than it would be to NASA.
 
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It’s an antimagnetic cover. It’s not a dust cover.

First, dust shouldn’t be getting into the case in the first place, and even if it did, the cover doesn’t seal the movement under it at all, as there are still openings that dust would get through.

It also serves the purpose of holding the movement in place.