They are very similar. The 28-29 had a bit more "curve" where the bumper is attached with the clamp. (i.e. to match the curve of the bumper bar.) The 30-31 are "straight" here and the bracket in a plan view (i.e. looking down from above) describes a long curve right angle.
I surmise it might be possible to "bend" a 30-31 bracket a bit where it attaches to the bumper and make a conversion.
ALSO - and for your shopping success - if you're attempting to replace a single lost or bent bracket - BRING THE OPPOSITE HAND bracket with you if you can so you can compare curve and match before your purchase.
I was at Epping Antique Auto Flea Market and was surprised by how many "near misses" I saw. I mean up to 3" overall size misses.
I suspect there are some Model T and Model AA brackets out there in both year range designations that are quite similar to the A brackets - but don't match the A car.
Joe K