1) Probably for 'scottish nail', one that could be clenched.
Noted as scotsem in Nottingham in 1273. It was frequent in Yorkshire from the early fourteenth century: 1313-4 Pro ij. ML scotnayl hoc anno iij s, Bolton Priory
1318-9 Pro dcccc. di. de spikings et scottesem, Bolton Priory
1371 Et in 10.m de Scotsomnail emptis pro celura, dando pro c. 5d, 41s 8d, York. The inference may be that ‘sem’ derives from the word for ‘nail’ or ‘rivet’, on record in the most northerly counties and especially in Scotland. It was used there of a nail that fixed together the planks of a clinker-built boat and may therefore have meant simply ‘scottish nail’, one that could be clenched. In that case the suffix ‘nail’ will have been added by clerks who were unfamiliar with the regional word. Later examples include: 1379-80 in scotsumnayle, 3d, Ripon
1434 In v. m Scotesemnailes, 5s 5d, York
1518 Item paid for ij M skotsym, ijs, York
1535 It’m twoo thowsand skott Semes, Stillingfleet
1537 scotsem nayles otherwise called lathe nayles, Sheriff Hutton.