those rings normally have that slit in them. the same rings are on the oil pump. the slit is so that you can install them as they don't have any stretch, they aren't rubber.
You put rubber o-rings on the piston? Thats fascinating, did you regain reverse?

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg