I think it's likely legit, Glover has mentioned a Mocking Bird in interviews before as one piece of gear he once had (he is not a collector). The explanation why there are no pics of him playing it is simple: He didn't tour between '73 (post-Purple-MK II-split and '79 (rejoining Ritchie in Rainbow), sticking to solo albums and production of others as well as A&R work for Purple Records.(I believe there was only one gig for him during all that time: Butterfly Ball in London - and he played acoustic guitar and keyboards during that, leaving the bass playing to John Gustafson or Mo Foster.)
So it must have been a studio bass only. This bass is supposedly from '77 (Roger once said '76), that was smack-dab during his "long lost weekend" 6-year period when he had given up regular gigging (after having toured himself to ill health with Purple from 69-73). As a shape, the Mocking Bird sure looks like something that might have interested him (he doesn't shy away from outlandish basses) if you look at the basses he played with Rainbow immediately (79 onwards) after: TBird and (after that broke at the headstock) Ovation Magnum II, both of them in a natural finish btw which seemed to appeal to him at the time (besides being the fashion then).