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（The Forme of Cury|The British Library）

Take and make a foyle of gode past with a roller of a foot brode. & lyngur[ 2] by cumpas. make iiii Coffyns of þe self past uppon þe rolleres þe gretnesse of þe smale of þyn Arme. of vi ynche depnesse. make þe gretust [3] in þe myddell. fasten þe foile in þe mouth upwarde. & fasten þee [4] oþere foure in euery syde. kerue out keyntlich kyrnels [5] above in þe manere of bataiwyng [6] and drye hem harde in an Ovene. oþer in þe Sunne. In þe myddel Coffyn do a fars of Pork with gode Pork & ayrenn rawe wiþ salt. & colour it wiþ safroun and do in anoþer Creme of Almandes. and helde [7] it in anoþer [8] creme of Cowe mylke with ayrenn. colour it with saundres. anoþur manur. Fars of Fygur. of raysouns. of Apples. of Peeres. & holde it in broun [9]. anoþer manere. do fars as to frytours blanched. and colour it with grene. put þis to þe ovene & bake it wel. & serue it forth with ew ardaunt [10]. 

The Forme of Cury: A Roll of Ancient English Cookery Compiled, about A.D. 1390|Project Gutenberg

”of a foot”で検索をかけたら、わかった。

”uppon”らしい。upon の古い表記では uppon だったのだ。
そう言われてみれば、そうだったような気がする。
apple の方はそのまま残っている。