I agree that 23638 is likely 105th PA Volunteers, but I also noticed in Marcot (p61) is a 37th Massachusetts Volunteer Infantry rifle (23862) from the Paul Putty Collection. Although there is no further information indicated about that rifle, rifle #28759 is identified to Horace P. Clark, also of the 37th Massachusetts who mustered out in June 1865. According to a letter in Marcot by BGen Oliver Edwards, the commander of the 37th Massachusetts, to the Chief of Ordnance for the State of Massachusetts, he wrote that the 37th Massachusetts received their Spencer Rifles on 14 July 1864, which according to ordnance returns for that regiment indicates they recieved the Spencer's in a camp near Petersburg, VA. There is a Invoice of Stores dated 30 Sept 1864, at Camp in the field, Virginia, showing some 30,689 rounds of Spencer Rifle cartridges and other spare parts, which based on the quantities listed in the Army Ordnance Manual for 1,000 arms, figures only about 60% of the spares required per 1,000 arms, which suggests that the 37th Massachusetts may have only received about 600 rifles. We know according to Marcot, the last batch of rifles that were to be delivered to the State of Massachusetts were, with the permission of Massachusetts Governor Andrews instead delivered to New York Arsenal on 7 May 1864 so it may be that the 37th Massachusetts received part of this shipment of 1868 rifles (together with 1176 carbines recieved 9 May), rifles less than 60 days later. That letter is reprinted by Marcot, comes from the House of Representatives Documents, to Governor Andrews from BG George Ramsay, Chief of Ordnance, Washington DC. I am just wondering what might be the serial number range that is identified for any Massachusetts units for Spencer Rifles in 1864?