Interesting, but I think not.
IIRC, The B1550/1750/2150 series was built between 1988 and 1995 or 96. Mine is a 1993 unit, special ordered in June of 1993 because there were no Bi-Speed units around. Clearly, mine is not an early unit.
Moreover, my copy of the WSM, provided to me by my dealer in 2018, clearly shows the Bi-Speed gearcase integral to the front axle assembly and makes no mention of any Bi-Speed mechanism within the transmission.
Moreover, as a former design engineer and engineering project manager, it is inconceivable to me that Kubota would have gone to the expense of redesigning the Bi-Speed mechanism for two or three years of future production of the B1550/1750/2150.
Bi-Speed Turn was used on larger models as well. The last tractor that I saw with Bi-Speed Turn was an M6, about 10 years ago. I did not notice how the Bi-Speed Turn mechanism was implemented and it may well have been within the transmission, but I have never seen a B1550, B1750, or B2150 with Bi-Speed Turn mechanism implemented anywhere but within a gearcase on the right side of the front axle assembly.
I will believe otherwise when I see photos or other documentation of B1550/1750/2150 Bi-Speed Turn mechanisms anywhere but within a front axle gearcase.