@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 Code: 5c4140e5d45de86011a27a258d916280ab32d98277a986f68140a8f3b8802664 and the sha256sum of my eMMC image RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin is Code: be0a1c9fde0ccc281ef5337fdb31ea35332d32a51b40200fbff63803dc7fb5f0 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).
5c4140e5d45de86011a27a258d916280ab32d98277a986f68140a8f3b8802664
be0a1c9fde0ccc281ef5337fdb31ea35332d32a51b40200fbff63803dc7fb5f0
$ sha256sum -b RX-51_2009SE_21.2011.38-1_PR_COMBINED_MR0_ARM.bin 775fde934550e72dbe88404db390ad351fb409b1b9c1d22ecdeccf1fcaf5db07 *RX-51_2009SE_21.2011.38-1_PR_COMBINED_MR0_ARM.bin
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".