"The 1873 was the first Winchester lever action to have a steel receiver. But it was in .44-40, .38-40 and .32-20. It was never chambered for .45 Colt or .45 Schofield."
I'm not 100% sure, but I think that both the 1873 and the 1886 introduced steel receivers at the same time.
Prior to that, the receiver on the 1873 wasn't steel, but an iron receiver, which was more than suitable for the black powder in use at the time.