Strange looking top plate. Looks nothing like my R69S (ie, an actualy /2), so maybe it works for the '55-'60 models. I understand the large hole where the bearings go plus the two smaller ones way out on the edges, but those two largish holes near the middle looks unusual.
I would think that an Earles fork for a /2 ('61 - '69) might work on an early /5, but if this is from the '50s, there might be some fitment issues.
The top plate is for an R26 or R27. They have a rubber mounted bar system.
The swingarm has a curve on the right side which (I think) is twin only. I'll have to go downtairs and look at mine to tell the differance.
The single forks have less trail and use ligter tubbing than the twins. They are interchaingable at the steering head, and useing single forks on a conversion is a common way of getting clearance between the fork and the 247 engine.
Any 61 to 69 fork should fit a /5. You'll have to do some checking to see what, if any diff there is in the 55-59 forks.
The single and twin top plates are interchangable.
Haveing had a good second look at the pics, they are deffinatly from a single. There is no mount on the swingarm for the flat bar fender brace that the twins have.
As far as /2 forks on a /5, there is a guy on the /2 conversion list on Yahoo that did it and it seems he had to do some things to make it happen. What those things are I'm not sure off the top of my head but I'll see if I can find it.