I have the opposite problem, I have too much play. When i bought my F1 it had a series of different size nuts instead of the brake side spacer.
I ordered this part: 42313-323-010 which is too small. I think this is for a K Bike.
I've now ordered this one so I hope it fits! 42313-390-000
I found this in a post on the American site. Might be useful to you.
'74 550
Rear hub
010 HUB, RR. WHEEL 42601-323-030
Backing Plate
019 PANEL, RR. BRAKE 43100-374-000
Collar backing plate side
008 COLLAR, RR. 42313-323-010
'76 550FBrake Panel
003 PANEL, RR. BRAKE 43100-374-000
Hub
009 HUB, RR. WHEEL 42601-323-030
Backing Plate Collar
002 COLLAR, RR. (backing plate panel side) 42313-390-000
'78 550K
Collar Brake Backing Panel Side
001 COLR, RR. PANEL SIDE 42313-404-000
REAR BACKING PLATE 002 PANEL, RR. BRAKE 43100-393-781
002 PANEL, RR. BRAKE 43100-393-781 (replaces 43100-405-000)
002 PANEL, RR. BRAKE 43100-393-781 (replaces 43100-405-010)
Rear hub
007 HUB, RR. WHEEL 42601-404-000