We can implement the codes of Example-1.1 of Post#5, and we will observe that the UNO is running at 1 MHz frequency rather than at 16 MHz (I tested this Example). Let us power down the UNO and power it up again; we will observe that the UNO is running at 1 MHz speed. This results have convinced me to accept the CLKPR Register as a non-volatile EEPROM type register. It could be that the naming is not suited; but, the behavior of the register is an experimental fact.