Olivetti 3MB ST506-interface drive?

Does anyone recall an Olivetti ST506-interface drive with a colossal 3MB capacity? Apparently a full-height 5.25" unit with 4 heads and two platters. I'm just curious; I always thought that capacities either equaled or surpassed the 5MB of Seagate's ST506 after they introduced it, so I was a bit surprised to hear of a drive with < 5MB. For context, Acorn apparently used them during development of their external Winchester units for BBC micros (Acorn SASI board, Adaptec SASI-ST506 bridge, ST506 drive). Production units that I'm aware of had either a 10MB or 30MB drive fitted (BASF typically, I think). We're talking 1983, or maybe late '82, so considerably after the introduction of the original Seagate drive. The only Olivetti drives I'm finding mention of are a 10MB single-platter drive and a 20MB dual-platter drive. Acorn had close ties with Olivetti, of course, so I did wonder if Acorn acquired some pre-production drives - but it seems like a bit of a leap to go from a 3MB dual-platter prototype to a 20MB one. cheers Jules