Not sure why, but my parts book for the B2 and B3 LTD's does not show it or a PN for it. It is shown in the shop manual and called the "Throttle valve" but again is not listed in the parts diagram. Another mystery!
The shop manual suggests that the "Throttle valve" parts not be removed. Quote "Theses parts are aligned during manufacturing and if they are misaligned the engine may not run properly". It "might" be better in the long run to find a complete carb???