A dealer must keep 4473s for 20 years and then may destroy them. If the dealer goes out of business before then, any forms younger than that must be sent to the BATFE.
A 30-40 year old firearm would have a serial number and the maker would be identifiable. A trace would start by asking the maker where that gun was sent and those records would be quite likely to exist. After the first sale to a dealer, however, the likelihood of getting results on that trace would fall off pretty quickly. Each subsequent change of hands would be another multiplier of the odds against being able to find owners, identify buyers, etc.
Even if 10 years ago that gun was sold to a dealer and so went "on paper" so to speak, there would be no way of linking that 4473 record to previous ones at other dealers, including the original seller. Say Jim bought that revolver in 1973 from ABC Guns, in Leesburg, VA. He kept it 10 years and sold it to his pal Ed. Ed moved to Cincinnati five years later. Ed died in 1989 and his son, who lived in Arizona at the time, inherited the gun. Ed's son traded that gun back in to his local gun shop as part of payment on a new Winchester in 1993, and Bill bought it from that dealer on a 4473 later that year. The gun is stolen in 2000 and ends up found at a murder scene in 2015. All the trace is going to show is that S&W made that revolver in 1973 and sold it to a distributor who sold it to ABC Guns in VA. That's probably just where the trail ends. If by some chance the records from ABC Guns DO exist (let's say ABC went out of business in 1987 and sent in their records ... and somehow the clerks in the records vaults can dig up that one yellowed paper form from the millions and millions they've got stacked up in file boxes) all that gets them is Jim's name and address from forty-two years ago. The only possible way they'll ever trace that gun another step farther is if Jim is still alive and they can somehow find him, wherever he now lives, AND if Jim remembers to whom or where he sold that gun, and some info on how to locate that buyer...who's now dead...etc.
These things are possible, but take a ridiculous amount of man-power and luck to trace, and to what end? If the 2000 theft was reported, the serial number will probably turn up in a NCIC stolen gun search anyway, so all this tracing would be pointless. The police would then know who did own it 15 years before the murder, but that person had lost possession of it then, so knowing that gives them very little help.