We need Larry Redbird R33 to settle this. He has access to the records of all car assignments. He can confirm that NO (i.e. none) R27's ran on the C until mid 1989. All R27's were assigned to CI or ENY during from May 1987 to mid 1989.
Of course, the occasional fluke R27 is possible. But they never ran in numbers, nore were they permamently assigned.
This period of time I was regularly doing what I called then "subway surfing", or riding the rails for sheer fun, much to the detriment of my grades (I cut school frequently), at least once a week, sometimes 3 times a week. I took notice of every line and the cars that ran on them. Since my favorite line of the time was the A, I saw plenty of C/H service. During this time, nothing but R10's and R38's ever ran on the C line.
I can prove my uncanny memory from this period by stating some of the more bizarre car assignments of 9/87-10/87, which had R68's on the F, R46's on the N to Astoria, red R30's on the R to Forest Hills and green R10's on the K.