This has a 0.0002% chance in Minecraft.
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In case you wonder, I calculated it:
Spawner that close to base: ~1%
Skeleton spawner: 25%
otherside music disc 1: 3,1%
otherside music disc 2: 3,1%
0.01 x 0.25 x 0.031 x 0.031 = 0.00024%
two otherside discs: 0.031 x 0.031 = 0.096% or 0.1%