Is it now still safe to fire? No-one on the forum can confirm that just by looking.
This topic has been covered several times in detail on the Lee Enfield forum, by Peter Laidlericon and others with professional experience.

https://www.milsurps.com/forumdisplay.php?f=72

The basic rule is: since you do not know why it was marked as DP, you must assume that it is not fit to fire live ammunition until it has been thoroughly checked out and judged to be OK by someone who knows what to look for and how to check it.