The Rudder and hardware are the same in both of these assemblies.
The length of the mounting brackets are the only difference.

• Item#: SPDR-001-M MONO .21 Rudder assembly, as pictured
• Item#: SPDR-001-H HYDRO .21 Rudder assembly, as pictured
  Price: $39.95

• Item#: SPDR-002 .21 Rudder Blade, as pictured
  Price: $18.00

There are no pictures available for the items listed below

• Item#: SPDR-003-M Transom Bracket ONLY for 21 MONO
• Item#: SPDR-003-H Transom Bracket ONLY for 21 HYDRO
• Item#: SPDR-004 Control Horn ONLY for the 21 Rudder
  Price: $5.60
  
  
• Item#: SPDR-009 Teflon Rudder Bushing for 21 Rudder's
  Price: $1.36