A little further research - Page 118 of Setright's Bristol Cars and Engines - suggests more customisation but he refers to a Hemi head although it was polyspherical or semi-hemi. He also states that Bristol threw out the hydraulic lifters and substituted mechanical lifters but a bit of research on A series / Poly forums suggests that there was nothing unusual about solid lifters on stock US or Canadian 313 engines of the period.
So, as Stefan says, more shadowy rumours... If Setright could get it wrong only 15 years or so after it happened we will indeed have to wait for more factory correspondence to get a better idea of what really went on.
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