Serial number appears to be E19783 ??
If so it was made in 1974.
The "M below the serial number is an inspection stamp.
There will also be an inspection stamp on the left-rear of the trigger guard web and the Colt Verified Proof stamp of a tiny "VP" in a triangle on the left-front trigger guard.
Be VERY careful who you send it to for a re-blue.
Too many refinishers are no good at doing the ultra-high polish Colt Royal Blue.
When you get the gun back the stamps have been buffed off and the edges that should be sharp and defined are rounded like a bar of soap, the flats have waves in them, the holes are dished out, and the sides of the hammer blued.
Here's the only places I can recommend who are able to do a top quality Python re-blue without botching it......
http://www.fordsguns.com/
NOTE: Ford's USED to have a top reputation for being able to do a 1950's Python "wet look" Royal Blue.
A few years ago some members of the Colt Forum sent in Pythons and got back ruined guns.
They'd hired a new worker and he was a gun butcher. I don't know if the situation has been corrected or not.
Until I hear more, it's buyer beware.
http://www.gunbluing.com/
Glenrock is famous in the industry and is used by many custom gunsmiths wanting the finest blue possible for a top end custom gun.
https://www.customshopinc.com/
The Custom Shop IS NOT AFFILIATED WITH THE COLT CUSTOM SHOP.
Colt did list them as a possible source of Colt repairs after Colt stopped doing work on the older guns like the Python. Colt does NOT warrant their quality of work, they just said they "might" be a source of work for a Colt.
The pictures on the web site look good, and a couple of people report a good job. I have not personally seen their work.
http://www.ronsgunshop.com/
Like Turnbull's Ron's is a firearms restoration service who offer all the old type finishes. Since Turnbull apparently no longer does Pythons Ron's might be a good choice.
http://smithgun.com/
Smith is said to offer a good mirror Royal Blue type finish.
Note that these days you can have less expensive bluing or you can have good bluing.... Choose ONE.
Even the less expensive bluing is expensive and you don't get what you pay for from the cheaper places. You get a botched gun.
95% of a good blue job is in the hand polishing, and top workers don't work for minimum wage.
Top refinishers use LARGE diameter, WIDE, HARD polishing wheels, not small diameter, thin cloth buffing wheels.
Before sending a valuable Python off for a refinish, TALK TO THEM about what you expect.