In a 32-bit floating number with normalized mantissa and excess-64 exponent base 16, the number $16^{-65}$ denotes
The format you are using is not IEEE-754 but an IBM FP format.  The mantissa is fractional, so the binary representation 0001_0000_0000_0000_0000_0000 means 0.1 (base 16).  With a zero value in the exponent field, the exponent would be -64.  So you can see that $16^{-65}$ would be the smallest normalized positive value ($1 * 16^{-1} * 16^{-64}$)