Grundy
Congratulations on getting your hands on a neat old 75. Nailing dates of manfacture for 75 model Remingtons is just about impossible. At this time there are no records available to tie serial numbers to dates of manfacture. To complicate matters, 75's were made in batches and every time a new batch was made, they started over with the serial numbers. There are no records available to tell us how many batches were made or how many guns were in a batch. Furthermore we're not sure why they stopped and restarted the manufactureing process. About all we can go on are features that evolved on the guns over the years. For example, the first guns were built for the Egyptian government and were chambered in .44 Remington. These guns were sequentially numbered from 1 up to somewhere around 16000, give or take. These guns had no caliber designation on them anywhere. With the introduction of the Winchester 44-40 cartridge, the .44 Remington quickly became obsolete. Early 75's chambered for 44-40 had the caliber identification stamped on the left grip, either 44 or 44W. Later 75's had the caliber marked on the left side of the frame just ahead of the cylinder, then later on the trigger guard, left side. Since your gun no longer has the original grips it makes dating your gun a little tough. If there's no caliber marking on the frame or trigger guard, I'd have to guess that it's a early gun. Whether it's chambered in .44 Remington or 44-40 is a bit of a guess. It's possible that your gun is a early Egyptian model (.44 Remington) or a early batch number gun in 44-40. My guess, and it's only a guess, would be that the gun was made somewhere between 1875 and 1880. I know this probably doesn't help much, but there's just not much to go on with these old 1875's.
If your Remington interest continues to grow, I'd recommend that you consider joining the Remington Society. You'll receive a great quarterly magazine and will be among the most knowledeable group or Remington experts anywhere. Joining is easy and can be done online. Check out the "How To Join" page on this website.