Newbie question on markings

[Kermit Van Roekel]

I hoping for some help, the old story came true, found in attic, hadnt been seen for 50 yrs... double barrel SxS 12 ga, exposed hammers, stamped 1875 with serial number 51621, and D4...and a frame size 6 ( which is correct for the 12 ga). So, what I've found is everything except the D4. I think D is quality level? Gun's is great shape, clean...and I need to get it insured so a approximate value would be helpful- knowing condition is the great variable...thanks anyone! Kermit