I've just got 10x of them from Questicon .I figure they do science stuff so hopefully they are fit for purpose .
The worst class is about to use them
Incidentally , I saw a You-tube where a very stretched metal slinky was saved by grabbing one end and pulling it inside out . I thought it looked too good to be true and tried it on one of ours that was stretched . It wasn't easy to do but it actually saved it from the bin .....not perfect , but still very much usable .