Yes PBA# starts with "E" can support L5420. Difference between SLARP and SLBBR is stepping. Normally mixing CPUs with +/- 1 stepping is OK, but remember to install the lower stepping in socket 1.
Check the page below for stepping information of your CPU:
ARK | Intel® Xeon® Processor L5420 (12M Cache, 2.50 GHz, 1333 MHz FSB)