PR 1.3.1 added only a few updated certificates to PR 1.3. AFAIK no official flashable firmware image was made available.

Yet I somehow also have 2014 RX-51_2009SE_21.2011.38-1_PR_COMBINED_MR0_ARM.bin (200174197 bytes and sha256sum of: 775fde934550e72dbe88404db390ad351fb409b1b9c1d22ecd eccf1fcaf5db07)

If you really need it (normally from PR1.3 you'd upgrade to CSSU, which includes PR1.3.1 and more), you can download it from here. I'll probably delete the link later this evening.

https://www.bbmk.org/cloud/RX-51_200...ED_MR0_ARM.bin