The serial number indicates the gun was produced in the 1974-77 period and there were very few .45 Colt Model 25s until after 1977, so it's almost certainly chambered in .45 ACP. Because it's post-1957, it will have the model number stamped in the frame cutout - it's a Model 25 for sure, and probably a 25-2.
The caliber will be stamped on the barrel, but on my 25-2 in .45 ACP it just indicates ".45 Cal", which obviously doesn't answer your question. My 25-5, in .45 Colt, is stamped .45 Colt on the barrel. Putting all of that together, and based on the production date and the short cylinder in the photos I'm pretty sure it will be in .45 ACP
You have a real keeper there - my 25-2, which has the 6-1/2" barrel and Patridge sights, is the most accurate revolver I own and would very likely be the last of my few dozen S&Ws that I would part with. I'm not a big fan of moon clips, so I load .45 AR cartridges for it.
Edit: Forgot to add - Thanks for your service!