Tinmuk-
I did some research, and it appears that the collets I have are Mk. 1A type. It looks like I actually have a Mk. 1A without the down-travel power-feed after reviewing the below site.
http://www.lathes.co.uk/linley/index.html
http://www.lathes.co.uk/linley/page3.html
"Collets were held in the quill by a compression nut on the nose - and could be had in sizes from 1/8" to 1/2" in increments of 1/32". Early collets were of the ordinary split-from-one-end type, which limited the machine's usefulness as a light-duty vertical miller, the sideways forces on the cutters causing them to work loose unless only the most judicious of cuts was taken; in what must have been a recognition of the fact that so tempting a small machine would (unless locked in a strong-room) inevitably be pressing into service for other than boring accurate holes, Linley equipped the Mk. 1A with the vastly-superior Schaublin Type ESX collet; this type had slits running from both ends and was specially designed to provided a secure grip on the shanks of small cutters.
Linley had offered a down-travel-only power-feed head as an optional extra for some time, but with the coming of the 1A it was made standard - though, no doubt, they would not have refused to sell a machine without and not all 1As may be so equipped."
