Ａ１の『く』字型レバーは脚作動用のハンドポンプレバーで Ｉ 型後期には廃止されていますので説明書指示通りで宜しいかと。
Modification 189 was "To introduce plastic seat as alternative design to assist production and provide alternative manufacture," and was requested from the Air Ministry in February, 1940, being cleared for use from 14-5-40. Note the use (twice) of the word "alternative." The plastic seat did not replace the metal seat, so any Spitfire could have had either type, and, even though removing the seat is a swine of a job (especially once armour was added,) a replacement could be of either type. As late as February 1945, mod 1117 was quoting "if pilot's seat is metal or plastic and has a wall thickness of less than .15". " I wouldn't place a bet on it, but I would not be surprised to find, given the dates, and their well-documented problems, that the initial idea was to supply plastic seats to Castle Bromwich, while Eastleigh, being geared up for metalworking, from the start, stayed with the metal seat. Of course, if that was the idea, the bombing of the Eastleigh works would have thrown the whole thing out of the window.
防弾板についてはAlfred PriceのThe Spitfire StoryのAppendix Cで N3171 は無し、BoBからIFFと防弾板導入となっています。