I have no idea where you guys are going with this because I don’t really understand the issue here. As @Daniel_360 was saying… gray mouse and gray elephant shouldn’t be much of an issue, because size-wise you’ll know… both being gray is really not an issue.
If you just need a decimal point, an E, and a minus sign… just make up your own image. If you’re thinking about something that is actually useful… consider that blind people are probably way ahead of you… and your basic PAO 00-99 can be used for binary and thus to learn Braille and then everything you need already exists…
We’re talking memorization here though… this has nothing to do with mental math really.