In fact there IS grades on E-type bolts, and as you would expect on a British made car, they use the British Grading system.
Why would a British car use the USA grading system as used in USA and pretty well no where else?
A basic Mild-Steel of 45-55 ton yield strength is Grade R.
Thus for those into XKs will find many bolts with BEES 45R55 embossed on the head - BEES being the Brand, and R being the Grade, but to be double identified also the 45-55.
Other brand bolts such as AUTO-R, and GKN-R just show their brand and their 45-55 ton mild-steel grade R.
The next highest grade commonly used was 55-65 ton, shown as Grade T
Then the next grade not often used was 65-75 ton, shown as Grade V
Then the top grade in Automotive use, 75-85 ton was shown as Grade X.
Now in the early 1960s, Jaguar actually marginally improved the strength of their mild-steel bolts to now be grade S instead of R.
Grade S is one increment stronger at 50-60 ton yield.
Anyone who has restored an E-type will find mostly Grade S bolts, a few T and a few X.
All the smaller fasteners less than 1/4" dia are invariably ungraded-mild-steel, less than R, and don’t need to be identified.
So the pictured flywheel bolt, shown as GKN (brand) and X, is a 75-85 ton yield setscrew.
Someone else can work out how that equates with GRADE 8