Alastair,
I have fitted a pair of Kadee no. 58 "scale" couplers to an 00 Lima Syphon. These are N gauge couplers in a No.5 draft box. They have worked OK but I have not bothered to standardise on them because, like No.5s, they do not have a long enough shank to hide the draft box under the buffer beam while still having the inside back edge of the knuckle in line with the face of the buffers.
They would probably be great for HO.
Colombo
I have fitted a pair of Kadee no. 58 "scale" couplers to an 00 Lima Syphon. These are N gauge couplers in a No.5 draft box. They have worked OK but I have not bothered to standardise on them because, like No.5s, they do not have a long enough shank to hide the draft box under the buffer beam while still having the inside back edge of the knuckle in line with the face of the buffers.
They would probably be great for HO.
Colombo