Sub mapper 3, assigned to 1995 super 2-in-1 (2B20). It is actually a multicart header function, the registers in $6000-$7fff are added to set the external PRG base A17 and the corresponding mask bit.
So I judge that
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An additional outer bank register at $6000-$7FFF:
Mask: $E000
D~7654 3210
---------
.... S.PP
| ++- PRG A17, A18 if S=1
| +-- CHR A19
+---- PRG mode
0: PRG A17, A18=MMC3 PRG A17, A18 (512 KiB)
1: PRG A17, A18=PP (128 KiB)
As the outer bank register uses the MMC3 clone's WRAM interface, writes only have an effect if WRAM is enabled and not write-protected via $A001.