Strange marking...pic (bad pic)

Just wondering what "424 LU" means? It's stamped right by the firing pin of my m10 ross. added a pic but its a cell phone pic and terrible quality...

The 424 LU will be a part of the rifle's serial number, enough to match up the bolt with the receiver.

The full serial number on Ross Rifles in Canadian Service was stamped on the right side of the butt, right into the wood. After rifles were sold off, the wood often was sanded down smooth, removing the numbers.

The FULL serial would have consisted (in your case) of 424 over a line; under the line was the year of production. The LU would have been stamped a bit over to the right and indicates the letter-group for that number. Numbers repeated as the letter-groups increased. Before group LU would have been group LT; group LV would have followed. The numbers were stamped in lines proceeding from buttplateward to muzzleward; the entire number group was about 2 inches long on most rifles.

Often you can 'raise' the lost letters and numbers of a rifle through the use of an ultraviolet lamp ("black light")which responds well to slight differences in height and wood density. Give it a try.

Your rifle has been in a workshop at some time, at which time the rifle number was stamped on the bolt. In Canadian Service this is not common, but we are seeing it more and more on rifles which saw service in the Royal Navy and with the Royal Marines Light Infantry, now that we know what to look for.

Would you be so good as to post a couple of shots of the complete rifle? Any markings on the receiver ring?

This could be very interesting.

Hope this helps.
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Here are some pics. I haven't figured out what any of the markings are...

The bulk of the markings on the top of the barrel are the D/C/P Dominion of Canada acceptance proof and the "E" signifies the chamber was enlarged at the Ross factory to help the rifle accept out of tolerance (by Canadian ammunition standards) ammunition.