I can bring some more details: the problem definitely looks to be that parted 3.2 doesn't like the GRUB4DOS MBR (see dump contents I supplied at end of in my first post), while parted 2.3 is ok.
Indeed, when I reset the MBR with bootrec /FixMbr from a Win7 boot CD, everything is ok in parted 3.2. And as soon as I reinstall GRUB4DOS, no matter which version (0.4.5c or 0.4.6a), parted can't read the partition table anymore. Still, the partition table itself (0x1BE to 0x1FE) is totally identical in all cases. So something in the rest of the MBR contents, between 0 and 0x1BD, is not digested by parted 3.2 ... are you able to tell what ? GRUB4DOS being quite spread and used (Grub2 is too complex to configure and requires to have a Linux install, which I do not have), parted should compose with it like other Boot Managers .. shouldn't it ? Thank you for any hint/help or let me know if you need anything more, Erwin9