This is indeed, a very interesting question.
According to EBAE Rule VII.28.a - the percent sign does terminates the number indicator. The effect of the number sign is not terminated by commas, colons, hyphens, fraction lines, decimals, or slashes. However, after a space or a dash, the number sign must be repeated.
Depending on who you ask, you may find different answers about the letter sign after the slash. However,
after a great deal of discussion with a few other "experts" regarding a similar issue - The end decision was that the letter indicator is not needed.
Even if the percent sign was not there, the letter sign would still NOT be needed.