I found out a new piece of hard to acquire info. Here it is for future newbie reference.
When Marvel-Schelber ran out of factory issue carbs, they issued Universal models as replacements. These then had minor adjustments to make them direct replacements for out of stock models. The replacement for the TSX403 (HG) and TSX406 (late HGs & OC3) is the TSXU828.
I have one here off my Universal Trencher and thought it was wrong until my carb-rebuilder guy did some digging.
Good info. Is the rebuild kit the same?
From Dan Ewald: Yes the kits will interchange with TSXU series carburetors. The gaskets, float valve and throttle shafts out of a regular repair kit will work. The main difference is the throttle and choke shafts, they use clamp on levers that are adjustable and can be moved side to side.
The TXSU838 or 837 will work for the OC4 and the Oliver 550 Carb TSX811.
The new Zenith carbs listed for OC3 are the same way.
I will try to post a copy of the Zenith list. When Dan refers to OC4 above, he means the 3 cylinder gas engine model since the 4 cylinder IXB version takes a TSX406 same as OC3.