In order for the blade of your sword to fit a cavalry scabbard it needs to be a cavalry blade. Since it has etching we can assume it was a officer's blade. A cavalry officer's sword is often shorter than a regular tooper's blade but still between about 34 inches and 34.5 inches. That would mean that your blade is cut down. You haven't shown the whole blade so we can't see if that has been done.
I didn't find any information about Helvig in Thillmann's cavalry sword book, but I did for Hoppe. Thillmann has an example of an officer's sword with a blade that is almost 34 inches long. It is a rather high quality sword. Officers had to purchase their own sword and this one was a bit too fancy to wear into battle. It had the standard cavalry hilt, only the officer's presentation version, and not an infantry hilt as appear on yours. Your hilt does appear to be a nice one and not a budget version. The Hoppe sword had the beehive maker's mark and the etching, from what I could tell, was different.
Looking at your blade, it is in rough shape. It has been cleaned up after accumulating a lot of rust and corrosion. It also appears that some one tried to shapen it with a file or power tools and doing a poor job of it. The damage to the blade is not reflected in the hilt and would suggest that the blade was exposed to the elements for a while before combined with the hilt. It's possible that this happened during the Civil War. It's also possible it happened afterward but it seems to have been done long ago. It may have been done during the 1930's when an antique dealer was trying to scrape by during the depression and put together two broken items into a sellable item but it also may have been done towards the end of the war by someone in the South when resources were tight and had to be stretched as far as they could go. At this point I couldn't hazzard a guess.