Stabilisation of RUKI

What converted the RUKI sound change (s > s^ after /r/, /u/, /k/
and /i/) from an allophonic change to a phonemic change? I thought
it was the merger of PIE -st- after RUKI with PIE -k^t-, but
examples of the latter seem to be very thin on the ground. I can't
think of any off-hand.

Richard.