You can find another nice O ringed mount here www.rdent.com/pages/carbmount.html. They carry lots of other stuff including the washers and alternative rubber washers - and this interesting advice:

Installation Tip: Use the rubber cushion mounts on the top carb studs (this is where the thackeray washers tend to fatigue and break), and use the thackeray washers on the bottom carb studs where the inevitable fuel leakage from the carbs will cause the rubber mounts to soften and fail."

The interesting bit here is that the plate is thin, and the "O-rings" are actually two half-round rings molded onto the plate to make a thinner assembly, almost like a single fat O-ring. This is significant for the MGA Twin Cam where the air cleaners sit very close to the inner fender.

On 17 July 2017, Mike Ellsmore in Victoria, Australia wrote:
"I have had no issue with Thackeray washers, nyloc nuts and soft mount kits over 7 years (see photo of installation)".


Bits still appear to be readily available from local Weber dealer:
http://www.weberperformance.com.au/product_info.php?cPath=115_101&products_id=237
https://www.weberperformance.com.au/index.php?cPath=115_102