RAM speed problems

So I have a ASRock P67 Extreme4 Gen3 Mobo in my desktop. For memory, I am using 2 Mushkin Enhanced Stealth 8GB 1600 MHz. I have been using this configuration since 2011. Over the last couple of days, I upgraded my GPU and system SSD, and reinstalled the OS, which is Win 10 Pro. Initially, I had some problems with the installation, related to the SSD. I had to end up clearing CMOS, which of course set everything back to default.

When I installed the Mushkin sticks the first time years ago, the mobo read it as 1333 MHz. I had to manually adjust the frequency to 1600. Once I did that, the system ran just fine, no problems. So when I cleared CMOS this time the RAM frequency went back to 1333. I went into BIOS to adjust it manually, as I had done in the past. And that's when I discovered that UEFI doesn't list any DRAM frequency above DD3-1333. Long story short, how can I get the system memory back up to 1600?