For those without access to my letters to Lord Lyon I offer my questions/comments to Lord Lyon and his reply.
The arms of Thomson of That Ilk, as described in the Workman's Manuscript (1565-1568) were never included in Lyon Court registrations. however, successive Lyons have accepted the existence of an unknown Chief and granted arms to Thomson Petitioners as indeterminate cadets with a central theme based upon that Armorial.
David Sellar, Lord Lyon King of Arms, replied on 17 June 2008. "It is true, as you write, that many Thomson arms have been registered on a common theme, but this reflects the general rule in Scots heraldry that those bearing the same surname, whether related or not, are granted recognisably similar arms. It does not imply, as you suggest, that successive Lyons have accepted the existence of an unknown Chief." A further explanation was given on 30 July 2008. "The Workman Manuscript describes a Thomson coat of arms as 'Thomson of that Ilk', the only place I believe, where such a designation occurs. Some later hand has added 'of Gourlabank' to this. Stodart (Scottish Arms) in the 19th century noted that these were the arms of 'Henry Thomson, Lyon King of Arms, 1504-12: he held the lands of Kellar, Farnyslaw, etc.., in the barony of Dirleton ....', but added his opinion that 'the designation of that ilk is complimentary', that is, I take it, intended to compliment or flatter Henry Thomson. I have just noticed further that J.H. Stevenson (Heraldry in Scotland, 1914) in his list of holders of the Office of Lord Lyon at the end of his second volume (supplied by Sir Francis Grant) designs Lyon Henry Thomson, 1496-1512 as 'of Kellour'."
Notice how he apparently did additional research to correct my mistaken assumption about a central theme. Our American government officials could learn a lot from Lyon's courteous answers to my questions.
Tom