Sony could likely have challenged Linn as the latter patented what was very clearly pre-existing technology. The
Linn Wikipedia entry suggests their bearing design dates from 1971, so five full years after the remarkably similar Sony was a commercially available product. It would be interesting to see if there is anything similar before the Sony. It wouldn’t surprise me as it is a logical approach to the problem.
Yes Sony or Thorens could have challenged the bearing design and my expectation is that the Tiefenbrun's would have conceded without it ever going to a hearing.
Hamish Robertson initially intended to challenge the patent based on prior use
"Following publication of the Tiefenbrun patent in May 1975, an opposition was lodged by Fergus Fons Ltd and William James Robertson, better known as Hamish Robertson on various grounds eg that what it claimed as new, was in fact old keeping in mind what had been shown to the Harrogate Audio Festival in 1971. They also opposed on the grounds that the idea was 'lacking in inventive step' over what was already known to engineers. A further ground of opposition was that the invention had been 'obtained' from Hamish Robertson, that is to say was rightly the invention of Hamish Robertson and not Jack Tiefenbrun."
The problem that Hamish Robertson had was that his own advertising claimed a unique point bearing following over a year of development.
This would allow Jack Tiefenbrun to argue that his point bearing design was special and those by Thorens or Sony were just mundane ordinary point bearings.
It would have been possible for him to attack the patent based on this but he would have had to argue against his own literature and also come up with an explanation why it would have been unreasonable for Jack Tiefenbrun to believe the literature was accurate. This would have destroyed his credibility so it appears he gave up on this when it came to the hearing.
The Patent Officer was not satisfied that Jack Tiefenbrun had defined what was special but it appears this did not create any grounds to throw out the patent.
It is clear from the patent coverage that the Ariston RD11 was based on a prototype turntable developed by Ivor at Castle. This would have been done with the help of Castle employees.
Outside the hearing Ivor has named former Castle staff who helped him with the development. "The design benefited from the input of my late father who designed the patented single point bearing and from the key engineering staff at Castle Precision Engineering, my late father’s company, including John Cross, Bob Hamond, George Borthwick and the late Russell Christie and Edgar Clumpas who all enthusiastically helped me with this ‘lunchtime’ project, along with many other employees at Castle". This implies that the Tiefenbruns would have been able to call on many witnesses at the hearing if true.
http://www.positive-feedback.com/pfbackissues/0706/Ivor.7n6.html
Also elsewhere Ray Collins, a former Castle employee (and later Ariston Acoustics) not named by Ivor above told Nigel Pearson that he helped Ivor with the development which provides some independent corroboration that Ivor did the early development. "Ray used to do listening tests with Ivor. Ray said he couldn't hear the differences and would humour Ivor by saying he did".
https://www.diyaudio.com/community/threads/sound-quality-vs-measurements.200865/page-777