There are aftermarket belts and there are aftermarket belts.
MTD in particular and Murray used belts that are not the standard M (3/8") A (1/2") & B (5/8") cross sections or have different Height to width ratio.
Some aftermarket belts are exactly the same as they are made in the same factory the only difference being the colour of the cotton cover.
Usually from companies like Oregon , Rotary, Stens , Thermoid and Gates these are marked with the OEM part numbers they are direct replacement for.
However because these are not standard sizes or length they have low volume production so at best 2/3 the price of a machine branded belt.
Then there are belts that will fit around the pulleys and get marketed as "aftermarket replacements ".
You can get away with using a std B ( 5/8") section to replace a CUB CADET 21/32" section belt, however it will sit 1/4" deeper in the pulley V groove which is the equivalent position of an original belt that is worn 1/2 way through it's full service life. Because it is sitting deeper in the pulleys it will be slacker so it will have to be a flea's fart shorter to run at the same tension.
Then there are things like the size and placement of the reinforcement chords.
If a belt has to go through a couple of 180 deg bends, it needs really thin chords that are a little deeper towards the middle of the belt.
If it twists then it needs thin chords that are almost at the outside of the back of the belt.
If it gets very high shock loads it needs thick chords.
When I started out, belts was the hardest thing to get my head around.
I would often find my suppliers had 3 or more belts that were all the same exterior sizes but drastically different prices.
Eventually I found out this was because they had different rubber coumponds and different chord size & placement.
The big problem is Greedy George packs his suitcase with 30 of the most expensive mower belts then hops on a jet to China, Belaruse etc etc etc.
He goes to I fool You manufacturing , plomps the belts on the table with instruction to make 10,000 of each belt that look exactly the same as cheap as possible.
So they make exact external copies using junk rubber, cheap cotton or pollyester chords and usually a polly cotton cover.
Greedy George leaves the brothel he has been staying in for a week and takes his 300,000 counterfit belts to the shipping companies , flys back to the US, OZ, where ever then sells his garbage into the wholesale market and you end up buying them from a discount tool store, Evilbay, Amazon etc for 1/4 the price of the genuine belt.
Wholesale belts are shipped in slings with no markings other than what is on the belt itself. It is the distributor or retailer who puts the sleeve on the belt with the consummer information.