The H072 has 17 spline axles. To my knowledge, the last year they were installed under a GM pickup was in 1971 on the 3/4 ton and higher rated pickups/trucks. It's ancient and was used for years, dating back to the mid-late 40s if I remember correctly. It was full floating axle with 8 lug hubs. It had both a removable rear cover and dropout center section. It's very distinct looking. If you have this, or something similar to it, you're probably going to have a hard time finding parts for it and it's going to be expensive. I priced a ring and pinion for one a couple years ago and it was $430. You could call Randy's Ring and Pinion in Seattle, WA and they could help you solve your question. www.ring-pinion.com
My 72 Blazer has a good 'ol 12 bolt and it has 30 spline axles. What year truck did the axle come from? If this is a 12 bolt then somebody, for whatever reason, may have cut a funky spline into it.