(i). There is no doubt that menstrual blood has differences to peripheral blood, but the extent of those differences and their testability are open to debate.
(ii). Whether tested blood is designated as menstrual blood rather than peripheral blood depends on the tests undertaken. Under some testing, menstrual and peripheral blood would not be distinguishable. It depends what you are looking for. In other words, it is possible it was menstrual blood and this fact was overlooked because it was not tested for.
(iii). To implicate Jeremy, the silencer need not have been contaminated with Sheila's menstrual blood, or even her peripheral blood. It was frankly admitted by the FSS that the blood tested could have been that of Robert Boutflour, Jnr.
(iv). I know that in response to (iii), the pro-guilt side will ask how the family could have known Sheila's blood group, but I will point out that they could have known it, and furthermore in the alternative, they need not have known it at all as any sort of human blood found in the silencer potentially implicates Jeremy due to the location in which the silencer was found.
(v). Finally, there is the fact that the chain-of-custody of the silencer did not include satisfactory measures to minimise contamination. Even in the 1980s, it was protocol to seal evidence in sterile bags or containers for that purpose, when and where practicable. The silencer should have been held or conveyed in a sterile, airtight bag or container. It wasn't. At any stage, cross-contamination (accidental or intentional) could have occurred with other bloodied articles. It's also overlooked that there is a degree of subjectivity in blood type findings. Biochemical interaction, even involving simple heat, can affect blood results.