@jdb2 if you think the PR 1.3.1 links you found are from a trusted source, then you can share them, maybe it will be useful for other people. The CSSU includes PR 1.3.1 so flashing PR 1.3 would work too.

The checksum of PR 1.3 and 1.3.1 should be different, otherwise it would mean it is PR 1.3.
The sha256sum of my PR 1.3 image RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin is and the sha256sum of my eMMC image RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin is I also found that the PR 1.3 is hosted on the domain name used for mirrors: http://maemo.muarf.org/tablets-dev/nokia_N900/ (but I don't know if the people who own the server are the same as maemo.org).

By the way, it looks like there is an error in the description of the PR 1.3 UK firmware, it is written "PR 1.3 version 10.2010.36-2" instead of "PR 1.3 version 20.2010.36-2".