PR1.3 kernel version which is in deb package kernel_2.6.28-20103103+0m5_armel.deb has string:
same version of deb package is on repository.maemo.org and on downloads.maemo.nokia.com.
and same version of zimage which is in that deb package is in PR1.3 and PR1.3.1 fiasco images (RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin and RX-51_2009SE_21.2011.38-1_PR_COMBINED_MR0_ARM.bin)

So check that you have that version string in file /proc/version. If not then you not have PR1.3 kernel.

With multiboot (which flashing kernel at every boot) I mean that it could use another and maybe old version of stock kernel.