I had never seen them before either - and haven't seen them again anywhere! I googled this: This battery size is most often used in small devices such as laser pointers, LED penlights, powered computer styluses, glucose meters and small headphone amplifiers. These batteries are not as popular as AAA or AA type batteries, and consequently are not as commonly available.
Some models of alkaline nine-volt battery may consist of a bundle of six LR61 cells inside,[4] connected together by welded tabs. These cells are somewhat similar to AAAA cells and can be used in their stead in some devices, even though they are 3.5 millimetres (0.14 in) shorter. These cells are often unmarked so special care must be taken with polarity.