I believe that upon receiving the 2nd silence at the Lab' on the 20th September 1985, and thereafter, that blood expert John Hayward would have been able to distinguish between the same blood group activity shared by both Sheila Caffell and Robert Boutflour...
The second silencer ('DRB/1') was not examined for the very first time until 25th September 1985, and by that stage, the blood group activity (A, EAP BA, AK/1 and HP 2-1) had already been found inside the 1st Silencer (DB/1), and analysed (12th, 13th, 18th and 19th September 1985). As such Hayward and Howard must have known that any blood found in the 2nd silencer (DRB/1) that wasn't examined until 25th September 1985, could not possibly belong or have originated from Sheila Caffell, considering that her death was supposed to have been a one gun crime, and that by 20th September 1985, the experts at the Lab' must surely have known and been aware of the fact that the blood group evidence obtained from the 1st silencer (DB/1) was worthless because Cook had exposed that silencer and it's contents to the harmful and damaging affects of Superglue fumes which were scientifically known to be damaging to scientific tests involving blood grouping evidence!
Hayward had no option but to say that the blood found in the second silencer could have come from either Sheila Caffell or Robert Boutflour, because of the existence of the 1st silencer which had blood inside it (albeit worthless evidentially)!