This is the underside of the Adaptec Controller Board, still looking in pristine condition despite being nearly twenty years old!
The difference between the ACB4070 and its predecessor the ACB4000 is the recording density. ST506 hard discs use either MFM (like ADFS floppy discs) or RLL encoding. With MFM, you can get 33 sectors per track and with RLL, 47 sectors per track.
The ACB4070 supports RLL and MFM but the ACB4000 only supports MFM.
The Seagate ST225 in the Technomatic Winchester Disc is a 4 head, 615 cylinder drive. All ADFS discs have 256 bytes per sector (although the Adaptec controllers can apparently support 512 and 1024 bytes per sector as well), so the disc size using RLL encoding on an ACB4070 board is:
4*615*47*256 = 29,598,720 bytes, or 28,905KB or 28.23MB.
It might not sound a lot compared to today's hard discs, but you try filling it up with Beeb programmes!