I am not sure if they are the same, I will check the two that I have when I get home and see if they are removable. I bouthg them in a "package deal" with my engine so I put them in the garage looking for someone to take them off my hands.
The A4ld has a removable bellhousing and it is simiply a C3 with a OD added, so the C3 should have a removable bellhousing as well. I don't know why you would want to though, because all C3s and A4lds are fairly weak. There does exist a bellhousing to put a C4 behind a 2.3, however it is fairly rare. It only came in certain models of pintos during the 70's. I see the bellhousings come and go on ebay.
My interest is not in the C3 or the A4LD, it's in the bell housing of the C3 and whether or not it will interchange with the C4 tranny or if the elusive 2.3/C4 bellhousing is the only option.
No, the C3 bell will not work on a C4 that I know of. The bells are an integral part of the front pump support and retention, and end play adjustment, etc, and are not related.