I remember the huge lightbulb that went off in my head when I discovered that number forms are a form of synesthesia.

I've had number forms all my life. I never knew where they came from or why, so the way I see numbers finally made sense when I found out that it was part of a condition I've actually heard of before.

I also have some weak chromagraphemia, but the number forms are much stronger.

For example, here's a rough approximation of how I see the numbers 0-100. It's not perfect because it's hard to do suble slopes in text.

                                                                                                              69 70 71
                                                                                                      66 67 68        72 73 74
                                                                                              63 64 65                        75 76 77
                                                                                      60 61 62                                        78 79 80
                                         29 30 31                                     59                                                     81
                                        28      32                                    58                                                      82
                                       27        33                                   57                                                       83
                                      26          34                                  56                                                        84
                                      25           35                                 55                                                         85
                                     24             36                                54                                                          86
                                     23              37                               53                                                           87
                                    22                38                              52                                                            88
                                    21                 39                             51                                                             89
    10 11 12 13 14 15 16 17 18 19 20                    40 41 42 43 44 45 46 47 48 49 50                                                              90 91 92 93 94 95 96 97 98 99 100
    9
    8
    7
    6
    5
    4
    3
    2
    1
    0

This also applies to years, by the way. Just add 1900 to all these numbers and you have my visualization of the 20th century. I find it amusing that the Great Depression is a downward slope... but so is the boom of the '80s.