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I'm an old survivor of those years.
Owned every ac model from R5 to the last RD 400 except DS7 and your US model 400 Daytona.
Every RD with what you call coffin tank has had disc brakes in the rear end.I sold a few of these hubs to the USA quite some years ago because Yamaha never sold spoke wheel models in the USA.
When new Yamaha sold them in the old world with both wheels.Cast ones had a higher price but as they were the modern way many buyers chose them instead of spoke wheels

fiy
if that would have been "eaten" by the rotor you would have found it in form of metallic dust

funny
just had a search session about other stuff and by mistake found this offer

https://www.2040-parts.com/aluminum-dg-street-performance-swing-arm-for-yamaha-rd-250-350-400-r5-ds7-i1067208/

I did it twice.
When I was young my caferacer based on a 351 RD350 was often the only vehicle.
When I killed the engine one of the last times I did buy a crashed 1st version RD 400.
 So I thought better riding with the slow 400 engine than walking.
But it wasn't that easy like with the other RD engines.
 Its very long time since that repair,but I remember I couldn't get the engine into the frame in one piece.If I remember right,I left the heads off.Also I found out later I couldn't get the cylinders off,I cut off a little piece of the upper cooling fins at the inner rear edges.
   Also the top rear mount gave me trouble so I just cut it off.
 No good idea,but as said I was in a hurry.
Exhaust was a mix,I used the 400 headers because of the different bolt distance and welded them R 5 style to the stock 350 pots.Only shortened the 400 headers.
 That way-with this hybrid I had one of my funniest summers on my 351.
I didn't change anything else,so carbs,airbox was still the slightly modded 350 stuff.
 Doing the testride I was quite surprised because I hardly recognized the powerband of the 400 mill.With this setup it almost revved like my dead 350 engine before.My friends at those times already were riding LCs and I had no chance with my ac350 to stay in touch on longer straights.
With this setup they could only get rid of me after quite some distance,so it did run a lot better than my 350 engine.
 But the rubber mounted 400 block gave trouble,the exhaust system wasn't made for the shaking 400 block being held rigid stock style.
 I had to refasten it often because of the shaking engine.The missing upper rear mount may have been also not helpful.I took it out again after some 5000 miles because the rubber bushings were totally dead.Then I put in another 400 block with the 4 mounts but shortly after that it caught some dirt in one carb and one piston melted.
 I never got it running again,at those times even pistons were hard to buy and parallel I started working as professionel bike mecanic.

thx
good to know
I'm almost reaching the start of the reactivation of my poor old caferacer based on a RD 350 model 351.I put in a fork of the first RD 400 after my last heavy crash with it.The rest of it are common mods of those old times.Tomaselli clipons and lampholders,R5 tank,Raask rearsets and a selfmade one piece seat/backcover.Engine almost stock.
I parked it many years ago after a piston gave up.At those times it was difficult to get parts at all and I started parallel to work as professionell mecanic for motorcycles.
 So I did turn a lot of screws combined with the daily test flights which are a must over here for the mec who did the job on the bike.
 So my RD got sort of forgotten.
If I make it I will let you guys here have a look at the reactivation job

Hi M in SC

are these the parts you mentioned ?

https://www.mikesxs.net/yamaha-xs650-damper-valve-pr-front-forks-77-84-oem-1t3-23170-00-00.html

If so,would they fit also in a fork of the first RD 400 which had the same diameter/34 mm as the first RD models ?
Asking because I'm told that these 400 forks were different inside from those pre 1976

               
                    greets    Richard