As you say, it is possible for hidden damage or correctable defect along the way. The tightness issue sort of reminds me of the old days, where one would run their piston / sleeve lapped motors, running rich (then called "4-cycle") accumulating at least 1/2 hour run time prior to flying. Otherwise, one could not maintain full speed by leaning out the needle.

If no damage or leak, perhaps the solution is to fly, running slightly rich, accumulate some time on it then recheck RPM?

Also, another possibility is if the OP lives at an elevation greater than 2,000 feet (610 m), there is a corresponding loss in horsepower, due to the thinner atmosphere. At for example, 4,300 feet (1 310 m), the air pressure is about 12.5 psia (0.85 atm) versus 14.7 (1 atm) at sea level. At 6,500 feet (1 981 m), it is 11.5 psia (0.78 atm). Change in elevation air pressure in atmospheres (atm) is proportional to the power produced. Figuratively speaking, figure on about a 15% power loss at 4,300 feet, a 22% power loss at 6,500 feet, and etc.

Therefore, the RPM at elevation will be naturally less than at sea level. If this is the case, then a 3,000 to 4,000 drop in RPM may be considered normal. This is why flyers at elevation will normally replace an engine with one the next size up. (Example, .061 for a .049, a .30 for a .25, a .46 or .50 for a .40, etc.)

The other option that sometimes works is to substitute the largest diameter prop permissible by the engine manufacturer. Propeller performance is much more noticeable at elevation than at sea level.