I've seen a lot of Finn Mosins from the 40's that also have the "D" stamp on them.
Keep in mind that the stamp indicated an expansion of the CHAMBER THROAT for bigger Soviet slugs and (IIRC) the famous D-166 200 grain Finnish slug, not a total reboring. You should slug your bore to be sure, but as an early M28 SIG barrel, I would expect it to be about. 309" or .3085" That would not have changed with the D chamber.
Also, do you see any other letters like an "A" or "B" or "C" around the receiver? It's possible the owner just never had it updated to "D" bore. He kept his old barrel when everyone else got the new ones, so he may have been stubborn even by Finnish standards.
Here's what Tuco says on the subject:
Q: If the rifle is not D stamped, is the rifle safe to fire using standard surplus ammo?
A: With M39 and WW2 M91 production the D chambering was standard, so many of these rifles will not have the D marking. That is due to the fact it was the standard so there was no need to mark such rifles. The only rifles that might cause a worry for lack of the D stamping would be the M91-24 and the M28-30 rifles. To be safe it is always a good idea to have the rifle checked by a gunsmith if there are any questions. Better to be safe than sorry.
http://mosinnagant.net/finland/finnfaq.asp
So my bet is with no "D" stamp, your 28-30, with its old barrel, was NOT updated to "D". A chamber casting will tell you for sure. The big issue is that the tighter chamber doesn't work so well with the Soviet surplus stock they captured in the Winter War.
Finding commercial ammo for this rifle, or surplus stock, is going to be less than easy. I would suggest seeking out the Ingman 150, which is .310" and should fit fine in the shorter throat. I would avoid Wolf or any eastblock rounds, particualrly the 180-203 grain rounds. Whatever you get, caliper the bullet and test the fit to make sure your ogive is not ramming into the throat.
Keep in mind you can handload the 54R with nothing more than a hand press ($20) and a hand primer, plus the usual components. The big cost is the good brass. Once equipped there would be no problems handloading with Lapua brass and keeping your ogive low. Load in the 148-160 grain range using .308" bullets. No shortage of those, so you're actually going to have a lot to choose from. Lee dies can be ordered with a .308" resizer die too. In fact they used to come with that standard. You might consider just setting this one aside till you can get set up for handloading. With some load testing and tweaking, you would likely be able to get it shooting true MOA. Or better. The M28-30 is one of the greatest military rifles ever made, and one of the most accurate. It's a neat one to have when some smartmouth makes a comment about pie plate commie rifles ;-)