Tony,
Rona summed it up above. The PC45 has the stirrer because it can burn 100% corn whereas the P43, 61, and 68 do not have a stirrer and the pots are more of a half of a bell curve instead of a flat bottomed pot. The P Series all can burn up to a 50% pellet and 50% corn mix straight from the factory without modifications or adding anything additional other than corn mixed in with pellets. The 45's stirrer simply keys into the end of the auger with two prongs into slots which the auger ends on the P Series do not have. The pot for the stirrer on the PC45 has a hole in an pot end plate for the other end of the stirrer to rotate in holding the forward end in place. Very simple design. Look at the first photos Overlook posted to see this. Next look at the ones right above here and you'll see it all pretty quick as to what is what. Then look at your pot and there it all is.
Looks just like your 61 but imagine where the auger end is that the pot bottom comes out flat from there on the PC45 where the newer pots come out very slightly and then curve up and out again towards the top. The stirrer on the 45 just sticks straight out from the auger end horizontally flat like the bottom of its burn pot.
I have burned this 50 / 50 mix in my 68 and PC45 both. Much the same as burning 100% pellets but with slightly more ash and mess from the corn bi-product or kernels. You also get a little more black greasy / sticky residue with the ash of corn inside everything vs. the light gray fluffy type with pellets. I ran my PC45 just like the 68 without the stirrer while burning the 50 / 50 mix with the same looking set up in the photos right above but with the pellet pot for the 45.
The pellet pot for the 45 has larger holes just the same as your 61's burn pot holes. The corn pot has pin holes about half as large. Overlook had an extra corn pot and since he is burning pellets only he has drilled the pot holes out to 1/8" whereas the corn pot holes are 1/16" or so. When burning corn less air goes thru the pot or I guess air is slowed for a more thorough burn. Corn is hotter to ignite than pellets. Since drilling the holes large in his corn pot Overlook can still burn a 50 / 50 mix as is just like your 61 can.
Besides the burn pot being different and not using the stirrer when burning pellets in the 45 the only other thing that is changed is the exhaust blower cover to a larger holed one like what's on the 61 letting more air go thru and exhaust. You can still use the smaller holed (corn) plate too and burn pellets only, but less air goes out the exhaust. When the 45 is set up to burn 100% pellets it is 50K BTU. I tried to break it down for comparison since I have both types or both stoves. Not much difference between the 61 and PC45. Same box, size, and all. It's the burn pot. stirrer, and igniter location that makes the only difference along with the PC45 being able to burn 100% corn which most don't do anyway. There are likely some CB and other slight differences but to get a quick gist of what I mentioned is what is really different.
Some say the 45 does not burn 100% corn great anyway and when the price of corn went thru the roof it was sidelined in production. Also since the P Series all burn a 50 / 50 mix I guess Harman saw no need to any longer justify the PC45 when the P43 and/or P61 are so closely related. To put it in bizarre terms. The P43 is a small mouth bass. The 61 a large mouth bass. The PC45 can be both with a hybrid of a catfish that eats about any junk. The P68 is a pig in the pond. LOL! Now ya get it!