That's really interesting pippa
It's certainly the earliest ring of that type I've heard of.
the source of my information is a classic article "Have you ever tried to sell a diamond? " which was an expose of the industry in the 80's.
DeBeers devised the "eternity ring," made up of as many as twenty-five tiny Soviet diamonds, which could be sold to an entirely new market of older married women. The advertising campaign was based on the theme of recaptured love. Again, sentiments were born out of necessity: older American women received a ring of miniature diamonds because of the needs of a South African corporation to accommodate the Soviet Union.
http://www.theatlantic.com/doc/198202/diamond
It's quite an eye opener.
This refers to the eternity ring being devised in the late 50's and early 60's but what you describe certainly sounds like one.
Are you sure it doesn't date from then? I'd guess your Grandmother must have been quite young in the 20s