Walter and Claude worked for Intamin/Giovanola at the time, the box track design was their own.
Walter and Claude worked for Giovanola before leaving to form B&M
Assuming that the box track was patented (I can't find a record of it having been so, but it might not be digitised), Walter and Claude would have been listed as the inventors but would not necessarily have owned the patent themselves. If they patented a box track design, whilst working at another manufacturer, it would become that manufacturer's IP. Walter and Claude would either have had to had an agreement when they left, that said that they could take any patents with them (which is unlikely), or B&M could have licensed the box design from Walter & Claude's previous employers. In the latter scenario, B&M still wouldn't hold this hypothetical patent. In much the same way that Sir Jony Ive is listed as the co-inventor / inventor on many of Apple's patents, but doesn't own them himself and can't use them at his current venture without a licensing deal.
Patents also have to be defended thoroughly if there is an infringement. You pretty much have to sue another manufacturer if they're using your patent, without license, otherwise you lose it.
I'm not doubting for one second that Walter and Claude invented the box design track and that it's their signature, but it doesn't seem as though it was ever patented, which is why a company that they worked at could use a box design after the duo had left, and why B&M could manufacture Nemesis in the same year. What B&M did/do patent fiercely, however, were/are the trains and other technical components.
From what John Wardley explained in an interview a while ago, the box track design can't be replicated by other manufacturers.
Z-Force, which opened in 1985, is considered to be the first coaster with a box track and designed by Walter & Claude, whilst at Intamin. If we were to assume that a patent was filed and granted for the track design, even allowing for a late filing granted in the year the coaster opened, it still would have expired in 2005.
John Wardley is a genius, but he's also a showman and not exactly immune to a bit of hyperbole. There are likely other reasons why the box track isn't widely replicated by other manufacturers, most likely down to cost (
@Tim) and a 'signature' look, but it's unlikely it was down to IP protection and it certainly wouldn't be the case now.
Comparatively, RMC's I-box track is patented. The inventors are listed as Alan Schilke, Fred Grubb and Dody Bachtar, but the assignee (owner) is listed as Rocky Mountain Construction. The patent was granted in 2013, so you won't see a coaster, made by another manufacturer (even if the inventors left to start their own firm), with I-box track until at least 2033; unless, of course, they grant a licensing deal.