Factory paperwork lists at least 4 different part numbers for HJ heaters:
1. 100171 This is the grey box with bakelite plastic lever on left front and the "K" emblem, first announced 9/12/50 hot water type
2. 100238 This has two fans, one for recirculation of air in car, the other blows hot air up into defroster slits. 11/3/50 has doors
3. 100141 Substituted for 100238 due to supplier issues, is the 1951 Kaiser unit - Memo from factory dated 4/4/51
4. 100302 Replacement for 100238, recirculating with defroster hook-up. First announced 12/21/51.
Only the "air conditioner" type heater (100171) is indicated as requiring the air duct on right side for proper operation.