Welcome Mark
Glad to see another Round tank being brought back to life.
Download the excellent model guide by SupeRDel to identify your model but 1973 almost certainly 'A'.
https://www.aircooledrdclub.com/smf/index.php?topic=650.0Obvious cosmetic differences between 'A & B'
UK round tanks (397), not true for other countries.
The 'A' had recycled/shared parts from CS5.
'A' square Pillion Pegs are right angle onto swinging arm. 'B' round Pegs inclined back to enable rider to kick back out of way, shared with coffin bike 1E8
'A' Bare switchgear, LH has all lighting controls, RH only has start button. 'B' Black switchgear, LH dip/high beam, RH lights on/off.
'A' tank badges are screwed on. B no badges instead decals. Check out the decals in model guide.
'B' front and rear drum have rubber inspection plugs to check brake wear. 'A' do not but you can see cast marks for provision.
'A' front mudguard elongated and 2 holes for mounting Razor blade - number plate.
Observed by me and others will contest and suggest more differences

. Of course over the years the bikes did see plenty mix and match and during production runs there may/will be overlap of bits.
Nothing originally on bikes saying A/B or even RD

, other than the Owners manual that came with bike.
Of course the frame number sequence can be used but Yamaha were notorious for confusion on this

Clear as mud
-Mike