Bummer, looks like no good suggestions for these Echo cats. I just bought a 352 muffler off eBay to replace a 310 muffler since the 352 I had didn't have a cat, but this one still had one. My saw had a C70 serial number, the parts manual shows a cat, but the C68 parts list doesn't.
I found it pretty easy to remove on a new 303t, I guess it helps to not run the saw first or the honeycomb hardens and crumbles. I expected an easy twist out on the 310, but it just made a mess.
I wonder if the cat could be heated and cooled slowly to keep it from hardening or would that not help?
Looks like I got the part number thing backward, the C70 manual shows no cat. I like the improvement. Unlike the equivalent Stihl models with a 10/10 exhaust rating, sooting up the muffler in one tank of gas.
Strangely the newer serial number saws seem to come with no cat, but that part number is slightly smaller than the previous run of 352's muffler which has the cat. After checking all the parts manuals for the 310 and 352, the cat is shown randomly, usually with the a300000803 muffler, but not always. My catless 352 muffler appears to have 2360 stamped on the front top, and the yamabiko symbol on the bottom left corner. The cat muflers I've seen say K10.
And I found proof that google doesn't work. I tried searching the same thing on different devices, one found one result while the other found what I was looking for.