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The 1200307L1 has both an 8-pin modular and 15-pin (essentially obsolete, common on old Mitel gear) T1 connector. The 1200307L2 is newer and eliminates the 15-pin connector. I'm not 100% sure in this case but generally when the part number changed from nnnnnnnLx to nnnnnnnEx it was due to Adtran switching to lead-free solder in manufacture for European RoHS compliance so the E2 model is likely the latest production and should be functionally identical to the L2 model. I'd go for either the 1200307L2 or 1200307E2 if the pricing is similar. The L1 is likely to be substantially older and electrolytic capacitors and the like may be starting to degrade.