Can anyone clarify the difference between tr87c and pv78 valve stems? I currently have the 90 degree angled stem (which I prefer), but don't ever remember choosing a model type. I'm about to replace my rear tire and tube, and have always bought the "angled" stem. I'm now noticing there are different models? I don't know how this has evaded me before. They both look identical on seller websites and there is no specific clarification regarding these two versions on line. Thanks.