As weird as it looks and as previously stated, it's original for the models 50 and 51.
The later 51-L have a hump or, if you want, a large bump made on the barrel, to be used as a ramp for the rear sight (see drawing below).
Only the model 50 and 51 had the chromed-lined molybdenum steel barrels. The 51-L "only" have a Moly-steel barrel.
Model 50 and 51 rear sight (from M 50 exploded view)
Model 51-L rear sight (usual set-up)
Both Models 50 and 51 were made on FN 98 actions, the 51-L used the 1640 action.
The FN actions are post '49, or if you better like, '50s "High Power" actions predating the Supreme (or "300") action.
These FN actions were supplied to H-S by both FN (for the Model 50) and Sako (for the Model 51, which at the time ('51 to '60), worked closely with FN).
These guns were offered as "kits" with bases scopes and rings and High-Standard used their own bases so as previously proposed, you will need a new hole to install a Weaver #45 on the rear bridge and # 46 at front ring.
From H-S records, model # vs Sears product code #;
Model 50 .30-06 / 583.90
Model 50 .270 / 583.91
Model 50 / 583.92
Model 50 / 583.93
Model 50 / 583.94
Model 50 / 583.95
Model 50 / 583.96
Model 50 / 583.97
Model 50 / 583.98
Model 51 .270 / 583.99
Model 51 .30-06 / 583.100
Model 52 .222 / 583.126
Model 51-L .30.06 / 583.5100
Model 51-L .270 / 583.5101
Model 51-L .308 / 583.5102
Model 51-L .243 / 583.5103
Model 51-L .30.06 / 583.5104
Model 51-L .270 / 583.5105
Model 51-L .308 / 583.5106
Model 51-L .243 / 583.5107