New old stock gear case cover, Honda part number 21611-952-770.  This part fits the following Honda lawn mowers.

HR21 SDA - HR21 1000001-1051093 - GEAR CASE / CLUTCH ARM

HR21 SXA - HR21 1000001-1051093 - GEAR CASE / CLUTCH ARM

HR21-5 SDA - HR21-5 1019070-9999999 - GEAR CASE / CLUTCH ARM

HR21-5 SDAM - HR21-5 1019070-9999999 - GEAR CASE / CLUTCH ARM

HR21K1 SDA - HR21K1 1100004-1127597 - GEAR CASE / CLUTCH ARM

HR21K1 SXA - HR21K1 1100004-1127597 - GEAR CASE / CLUTCH ARM

HR21K2 SXA - HR21K2 1200141-9999999 - GEAR CASE / CLUTCH ARM

HR21K2 SXAM - HR21K2 1200141-9999999 - GEAR CASE / CLUTCH ARM