Sorry I can't help you out with Benotto serial numbers. As for the tubing, I have four tubing scenarios, considering the bicycle is pre-1989, has oval tubes and had Columbus stickers:
1. Columbus Air tubing - This was the Columbus' true aerodynamic tubeset and was available in the early 1980s. The downtube and seat tube profile were more teardrop shaped than oval. That is the front edge was round and the trailing edge was pointed. I believe the seat stays were also teardrop shaped.
2. Columbus MS tubing - This tubeset was introduced in the late 1980s and used specially shaped tubes. The top tube was ovalized, while the down tube had a tapered leading edge and the seat tube was flared at the bottom. Perhaps the most distinguishable trait is the mismatched chainstays; the right side has an oval cross-section, while the left has a triangular cross-section. The other trait of an MS frame is a small diameter seat post, typically around 25-25.2 mm.
3. Columbus MAX tubing - Another late 1980s tubeset, it used slightly over-sized oval tubes and was intended designed for heavier/stronger riders. The top and down tube had vertically orientated ovals at the headtube that changed to horizontal orientation at the seat tube and bottom bracket.The seat tube is a lateral oval at the bottom bracket, but round at the top and uses a standard 27.2mm seat post. The chainstays and seatstays are also oval, while the forkblades are more teardrop shaped.
4. The final scenario is that Benotto used standard Columbus tubeset and shaped the tubes to their own specifications.
Regardless of which scenario fits, appears to be a high end frame. A fit with scenario 1 means the frame could be as old as the early 1980s. A fit with 2 or 3 restricts it to the late 1980s. Scenario 4 could fit any time frame. Please advise if any of the scenarios fit. If it is 2 or 3, I may have some Benotto specs for the late 1980s stashed away somewhere. Good luck.