I recently was looking for a "new" 124 mirror at the local LKQ, and I compared a 123 mirror with a 124, and although the two spring-points looked correct for size and distance, the bases are different. My impression was that the 123 base is bigger and won't fit a 124 receptacle.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC