I.
On June 2, 1977, a fire, followed by an explosion, occurred in a plant owned by Grenada Steel Industries, Inc. (Grenada Steel). In this diversity-based suit, Grenada contends that the fire was caused by a leak of acetylene gas through a valve on the cylinder that contained the gas. Alabama Oxygen Company, Inc., (Alabama Oxygen) sold acetylene gas to Grenada Steel. The gas was delivered in a metal cylinder, which was to be returned when the gas was consumed. The cylinder was equipped with a valve manufactured by Sherwood-Selpac Corporation (SSC) in December, 1972, utilizing a rubber o-ring seal around the valve plug to prevent leakage of acetylene gas. The valve’s design was based on a patent issued in 1965. The cylinder, with its SSC valve, was supplied to Alabama Oxygen in 1973. One year later, in 1974, SSC halted production of this type of valve. It was no longer being marketed when the fire occurred in 1977.
Liberty Mutual Insurance Company insured Grenada Steel. It paid $608,990.38 to Grenada Steel for losses .resulting from the fire. It then filed this suit, as subrogee, to recover from Alabama Oxygen and SSC the amount paid to Grenada Steel. Grenada Steel later joined in the suit to assert its own claim for damages not covered by its insurance. Since their interests are in most respects identical, we refer to both claimants jointly as Grenada Steel.
In the six-day jury trial, Grenada Steel proffered evidence that, following the fire, Regó, a competitive valve manufacturer, manufactured and designed an acetylene valve based on an alternative design and that SSC had itself manufactured a differently designed model. The district court excluded this evidence. Grenada Steel also introduced the testimony of an expert witness that the valve was defective. SSC countered with testimony that the valve’s design had a good reputation in the industry, its valves had been approved by Underwriters Laboratories, and the same design had been used in the industry for a number of years. Each side also presented contradictory factual and expert evidence concerning where the fire originated.
The district court, with counsel’s assistance, formulated both special interrogatories and a general verdict. In response to the special interrogatories, the jury found that the valve was not defective or unreasonably dangerous, had undergone substantial changes in condition after it was sold by the manufacturer and before the fire, and was not unfit for ordinary use for the purpose intended. It rendered a general verdict for the defendants. Grenada Steel does not assert that the answers to the special interrogatories were either inconsistent with each other or with the general verdict but only that they are “incorrect and do not square with the facts.” (Emphasis in Grenada Steel’s brief.)
II.
We start with an elementary proposition: in diversity cases, state substantive law applies.
Erie Railroad v. Tompkins, 304 U.S. 64, 58 S.Ct. 817, 82 L.Ed. 1188 (1938). Therefore, in determining the sufficiency of the evidence we follow Mississippi courts, which have adopted the American Law Institute’s Restatement (Second) of Torts § 402A as the appropriate standard of strict liability for product manufacturers.
Page v. Barko Hydraulics, 673 F.2d 134, 136 n. 1 (5th Cir.1982);
State Stove Mfg. Co. v. Hodges, 189 So.2d 113, 118 (Miss.1966),
cert. denied, 386 U.S. 912, 87 S.Ct. 860, 17 L.Ed.2d 784 (1967). In matters of procedure, however, such as the admissibility of evidence, federal rules apply. Fed.R.Evid. 1101(b);
Rabon v. Automatic Fasteners, Inc., 672 F.2d 1231, 1238 n. 14 (5th Cir. 1982);
Johnson v. C. Ellis & Sons Iron Works, Inc., 604 F.2d 950, 957 (5th Cir.1979).
The district court relied on Rule 407 of the Federal Rules of Evidence to exclude evidence that, after the accident, SSC and Rego marketed valves that were designed differently from the one patented in 1965. That rule provides:
When, after an event, measures are taken which, if taken previously, would have made the event less likely to occur, evi