Compass is required equipment per TC - but any approve compass should meet that requirement. I would not see a need for a "Major Alteration" 337 form.
A&P, log book entry, equipment list modification, and reference to W&B (likely no wt chg) W&B & Equip list should be current and in the Aircraft. Oh, and don't forget Compass Correction Card
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Herb R Harney
1968 337C
Flying the same Skymaster for 47 years
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