I've read in books and heard it also from other persons involved with these matters. I don't remember anymore it what book, I have too many :-))))). Yes, it has similarities to the German silver/alpaca alloys as you mentioned. It is a Soviet invention and often used as a base metal for electroplated objects. Maybe you have seen those Soviet tea glass holders in filigree as well as those small enameled salt cellars with a glass bowl inside? They are made of the same alloy and marked ЮММЕТ.
The content of the alloy is not 100% sure but it is claimed that a part of it would be silver the rest nickel or ? I cannot neither verify it nor show you any links probably because there are none at least I haven't managed to find any information. Therefore I stated 70/30 with "?".
The silver test with acid is not always fully reliable. When an item is electroplated the coat is 100% silver. You have to scratch very deep and through the silver coat to reach the base metal and that might damage the object. Moreover, when the base metal probably also contents silver the test might give a positive but incorrect silver result. What is absolutely sure is that your filigree basket is not silver but what the "jummet-alloy" is made of is not 100% sure most likely what I told you or very close to it.