I agree with Jeff............that skeleton would be broken down and scattered more at 18 years....rib cartilage dried, oxidized or eaten. Separated at sternum and spine. 2-5 years a solid estimate. I know of no pump action 45-70's, but that may have been a euphemism for levering in a cartridge.....in which case the rifle (which I cannot see) would likely be the big Marlin 80 (often called 81), or the Win 86. The Win 73 and 76 did not handle the 45-70, and the more common trap door Springfield was a single shot (and prone to jam with a weak extractor and an often pitted/sticky chamber.)...There were pump shotguns then, Spencer and Win too. .....FWB