Reading this thread with great interest as I was beginning to take an interest in UK road racing - spectating and marshalling - in the late 1960s.
From memory and looking at Claudio Sintich's book on the Thruxton Bonneville, 1969 looks to be just about the only year with an overlap of featherbed framed 750s and Commandos in the production class in the UK. For example, the results of the Hutchinson 100 at Brands Hatch August 10 1969 are shown as:
1st Mick Andrew 750 Commando
2nd Tony Smith 660 BSA Spitfire
3rd Rod Gould 660 Triumph Bonneville
4th Ray Pickrell 750 Dunstall Dominator <--Atlas engine, featherbed frame
5th Peter Williams 750 Commando
6th Percy Tait 760 Triumph Trident
Fastest Lap: Tony Smith 1m50.2s 86.57mph
Mick Andrew was riding for the well known London dealers and racing supporters Gus Kuhn Ltd. These were all amongst the top riders in their day.
Here is an extract from the programme introduction:
View attachment 8265
And list of riders
View attachment 8264
Earlier that year Thruxton Bonnevilles had won at the Thruxton 500 (no Nortons in top 6!) and at the TT where Uphill famously set the first 100mph lap for a production bike. Norton Commandos were placed 2nd (Paul Smart) and 4th (Mick Andrew).
The repeat race in Hutchinson 100 race in 1970 shows Atlas/Featherbed bikes entered but top 6 places are Triumph Trident, Norton Commando, Thruxton Bonnie, Triumph Trident, BSA Rocket III, Norton Commando. By 1971 no Atlas/Featherbed bikes were entered.
You can see many race programmes from the era here
https://daveriley.weebly.com/
As others have said Commandos and Tridents/Rocket3s then took over for a while at any rate. Most Commandos raced with success in the open 1000cc classes (non-production) were usually Seeley framed although some standard frames were used.
Such good memories for me.
Andy