Just the grip is different and as a result the modulating trigger on the grip.
‘Notenoughcash’ has the model numbers back to front in the previous post. The ‘b’ moniker is for barrel I believe.
I have the d-handle as I prefer that, but I have to say it’s the least ergonomic d-handle I ever used. I added some moulded rubber (sugru) to the trigger to make up for the oddly thin grip/bag ergonimics of it.
If you buy 2nd hand don’t get the 400! It’s utter junk as it doesn’t have the festool blade guide and is very inaccurate. No idea why it ever existed. The 420 is the one to get.