Patrick, I am happy to say that I now understand your question. Reading the "Subject," I was preparing a lecture on the evils of emotional relationships with inantimate objects.

OK, now for serious.

The marks I'm having trouble with are on the barrel flats. On the far left is a 13 over a 1. It may be a 15 over 1 but I think it's 13 over 1. What does this mean?

It is most likely 13/1 which translates to a bore of more than 0.719 and less than 0.730 at proof (allowances made for my remembery - which ain't what it used to be). ).730 to 0.740 would have been stamped "12". The 13/1 is a fairly common gauge and positively nothing to worry about.

The diamond with a 12 over C. I understand this is for 12 bore, but what is the C and when was this stamp used? (before 1925 I think?).

Actually, it refers specifically to a 12 gauge (see above for 12 bore) chamber of 2 1/2" or 2 3/4" chamber length - 2" chamber guns are so marked and a diamond 12/L was used for 3" and longer.

Last is the "1 1/8" mark on the far right. What does this stamp mean?

It indicates a maximum load of 1 1/8 oz shot and implies the level of proof pressure and intention for service loads. 1 1/8 loads are usually associated with 2 1/2" chambers and shell length where as 1 1/4 oz was generally for 2 3/4" chamber guns. Not invariably, but usually.

Any thoughts on the date and possible model and level of gun this is ?

Date I'm not sure of - someone else will fill us in I imagine. The gun itself would be a upper middle-of-the-pack boxlock ejector. Midland made guns of Original Quality grade from "best work with extra finish" to colonial-farmer grade. They were all sound and did the intended job; you got the grade you were willing to pay for. The Midland name commands Brand Value level three (BV3 in my shorthand). The Original Quality grade is six (OQ6) and Current Condition level looks to be about four (heavy use, but no abuse). So, BV3-OQ6-CC4 = $1766 for the USA mid-retail market for normal Brit-Continental 12 bore SXS guns of 1890ish to 1960ish manufacture. BTW, this is not a "price", but an idea of where the price (agreement between a willing buyer and a willing seller) might fall. Of course, this assumes it workes well and the photos are a true representation.

Hope this helps.

DDA