I appreciate there wasn't any request to see original factory drawings etc., but wanted to satisfy my own curiosity as to the meaning of all four Reg. Nos., and as I now have details it may be of some future benefit if Fred is able to add to the Board's archive.
The two separate shapes - the half and full circle, were both Registered twice, hence the four numbers - each shape being Registered under both CLASS 1 and CLASS IV ............... metal and pottery respectively. There is no mention in the archives of CLASS III (glass), and whether in fact these things were ever made in metal and/or pottery I've no idea. Designs being Registered in more than one CLASS isn't unknown but probably uncommon.
It's a tad academic to add four pix when two would suffice, but it does make sure that everyone can see that despite there being four consecutive Reg. Nos. there are in fact only two separate designs.
The Blue Book is correct to include only two Registration Nos., since there are only two separate designs - but why the author chose 694155 and 694157 I don't understand .................. none of them is CLASS III, and you'd think that the natural choice would have been Rd. Nos. ......4 and ......6.