1) These words may have the same meaning: initially they referred to split oak boards which were imported from the Baltic and used by coopers for barrel-staves.
They were brought into east-coast ports such as Hull, and moved inland along the rivers. In Yorkshire, there were numerous ports on the Ouse and its tributaries, for example, Boroughbridge, Cawood, Selby and York. ‘Clapholt’ is the earlier term and has a German origin, so ‘clapboard’ may have developed in England as a semi translation, possibly influenced by the fact that ‘clapboard’ was increasingly used for panelling. Early evidence includes: 1453 3C waynescotes 1 M 2C clapholt
1471-2 12C clapholttes, Hull
1526 pro iij.c tabulis vocatis clapburdes 14s 8d, York. Extracts from the accounts of the wood carvers who were working on the high altar at Ripon point to other uses for ‘clapboard’: c.1520 dicto servienti for carreyng of ij C. clapbordes de Ebor. ad Burghtbryg 8d
Item de craneagh de waynescot ad Hull 12d. Item pro craneagh de clapbordes ibidem 4d. In this case a Hull beer-brewer was the middleman. In New England, ‘clapboarding’ refers to the thin boards used for the outer covering of timber houses: it was a practice in some English counties and it seems likely that emigrants took that meaning of the word to America. The spelling ‘clabbord’ occurred in England and it became usual in America.