I've rebuilt hundreds of those 903s. If it's been jumping out of second on decel for any length of time, the clutching teeth are worn off, chipped off, and/or wedge-shaped; as are the ones in the slider. There is an easy fix for this if you have an electric die grinder. On the gear there are always only 6 clutching teeth involved or 8 at most, and you will find them, 4 and 4, at 180* apart. You simply back-cut those teeth, and the slider will find new teeth to destroy. But if you also back-cut all the slider teeth on that funky looking clutch gear sleeve (synchronizer) , then you can delay that for another decade or two.
While you're in there, you can check the input gear and service it in the same way, as may be required. If you can find new spreader rings get 'em. If not, put a bit of a stretch on the old ones.The low-gear is un-synchronized, so will look like crap. 10 minutes with the grinder and it will have a new pretty face. Another trick is to put the steel synchronizers in a lathe and deglaze the polished brake surfaces. And finally the higher the brass rings sit in the steels the better. If yours are below and into the steels it's time for new ones.
The 903trans is a LightDuty slow-shifting non-synchro'ed in low, unit. The A230 is also a 3-speed but is fully synchronized, and is an awful lot like the A833, but with one less gear. And you can shift the chitoutofit. Up AND down, even column shifted, if the dang thing is in good shape. IIRC the 230 has the same mounting pattern as the 833, which are both different from the 903. But also IIRC I have seen some bells dual-drilled for both patterns. They are all 1x23 inputs, so your clutch should work.
