Information on these short extractor models is pretty scarce, but it seems they appeared about 1967.
I don't think that the "L70" actually denotes the short extractor action, but the "70" is possibly the year of production.
I'm pretty sure I've seen an FN 98 actioned Browning also stamped with "L70" after the serial number.
I'll see if I can get some more info on the short extractor action and will post it here if anything interesting shows up.