Thanks, and yes, mine was the Bentley manual, not Haynes. Sorry about that. Lists 0.9 US quarts. So does the Moss catalog.
Under routine servicing, the Bentley manual states to periodically top-up the differential right up to the fill hole until fluid begins flowing out that same hole. That's always what I learned. And sure enough, it was 1.35 to 1.50 US quarts, as you say.
Agreed, TR4A IRS and TR6 differentials are the same unit.
Thanks
Bob