The checkering pattern looks very much like the factory Ballard checkering. At first I thought it might have had the original cheekpiece reworked on a factory stock, but Union Hill cheekpieces were not large enough to be reworked that far rearward.
Back in the day the Marlin factory would commission just about anything a buyer wanted, so it's possible it was factory, or just really high class work!
If it is factory the serial number should be stamped on the leading edge where it meets the receiver. Removal and inspection of the stock should reveal if the serial numbers match the gun.
I have a Union Hill #9 that has very odd checkering pattern and extremely high grade wood for a Union Hill. Most were very plain wood, unless ordered as an upgrade. I was surprised when I pulled the stocks on mine and found the serial numbers matched. I always thought it was wood changed from a higher grade Rigby, or #7A-1 rifle.-Vall
