Appears to a loss of bluing. Looks as if your bolt is hitting the cylinder.

 

That still looks like a lead deposit on the cylinder. I get the same thing on every revolver I own, SA or DA, blue, stainless or nickel. It is just the nature of the beast when you shoot lead.
It just takes a little more elbow grease to remove. I have had better results with Hoppe's #9 or Shooter's Choice than I have had with CLP. Lately I have been using a batch of Ed's Red that I mixed up for grins during the quarantine.

 

THAT IS HOT GAS AND LEAD BOUNCING OFF THE TOP STRAP WHEN YOU FIRE AND LEAVING DEPOSITS ON THE CYLINDER. NO BIG DEAL.

 

I get that build up as well and I just rub it with my paper towel and Hoppe's #9.

 

As the others have stated, just lead build-up; nothing to be concerned about. If you try to completely remove it after every shooting session eventually you will have blue loss.

Best regards,

 

A lot of the lead build-up at the front of the cylinder issues can be traced back to bullet choice. Almost all commercially made lead bullets are too hard for standard pressure loads. They tend to not obturate as they should and this causes a lot of blow-by and tends to create what you are seeing. Softer lead bullets usually solve this problem. I proved this to my satisfaction when I began casting my own bullets and lead on the front of the cylinder dissappeared.

 

To add to ColtSAA#G comment, the blow by melts the base of the bullet. The hot gas and lead is coming back from the top strap. Back in the the day BHN 12 was considered hard enough for 44 magnum loads. You didn't say if you are reloading. Many commericial bullets are a BHN of 15 or more. However commercial ammo is usually less than 12. So if your using store bought ammo I don't think blow by is your problem.

 

My 2020 3rd gen got the same thing from firing SJWC. It was just build up. It cleans off.