The HRS21 was from the early 1980s, so about the same age as my 1983 HR214... I believe the HRS21 had a GXV110 engine, while the HR214 had a GXV120. The HRS was side discharge, while the HR21 introduced rear discharge back in 1978.
The HRS21 I just picked up came with the side discharge bolted on, but also cam Dwight the bracket that bolts into the same place as side discharge where the grass catcher would hook to. Just didn’t come with grass catcher.