If the radix is 15 or higher, we can't use the letter `e' to mark the exponent because `e' is interpreted as a digit. When Calc needs to display scientific notation in a high radix, it writes `16#F.E8F*16.^15'. You can enter a number like this as an algebraic entry. Also, pressing e without any digits before it normally types 1e, but in a high radix it types 16.^ and puts you in algebraic entry: 16#f.e8f RET e 15 RET * is another way to enter this number.
The reason Calc puts a decimal point in the `16.^' is to prevent huge integers from being generated if the exponent is large (consider `16#1.23*16^1000', where we compute `16^1000' as a giant exact integer and then throw away most of the digits when we multiply it by the floating-point `16#1.23'). While this wouldn't normally matter for display purposes, it could give you a nasty surprise if you copied that number into a file and later moved it back into Calc.