I did some more research on pistons.
OEM pistons with valve cutouts are not sold any more. They are superceded by simple flat top pistons without any recess or cutout.
I dug out the box of the 0.5mm oversize piston set I ordered 2 or 3 years ago for my SR42. The part number is 12100-31D00-050. And the same piston set is sold now by CMSNL for SR41, SR42 and SR43.
Conclusion - flat top piston without valve cutouts will work on all DR Bigs be it SR41, SR42 or SR43.
Thanks to BigBob on drbig.info who led to the right tracks.