Fact is, there is a cowbell on "Reaper." If you listen closely to it on oldies radio, you can make it out in the background. But it was an afterthought. The song was recorded without it, and was added as an overdub at the last minute. According to former BOC bassist Joe Bouchard, an unnamed producer asked his brother, drummer Albert Bouchard, to play the cowbell after the fact. "Albert thought he was crazy," Bouchard told the Asbury Park (N.J.) Press in 2000. "But he put all this tape around a cowbell and played it. It really pulled the track together."
There's more on the lingering effects of the More Cowbell SNL bit from a Washington Post story here.