Yes, you're quite right.
The red vertical trim behind the left ear on the MR casts seem to meet the orbital band at a very sharp angle and is quite fat whereas the PP2 based lids seem to have a softer meeting point plus the vertical strip is thinner. Just had a quick look at a couple of helmets and this BM does seem to have some PP2 features on it and dent seems more reworked PP2 than MR by a stretch. I know the older BM helmets, with incorporated wavy brow, seemed to have been regarded as on the smaller side. The earliest PP2 casts I've seen are very much MR sized so just wondering if that could be what this is given your feeling that this was in that MR sized range?