Norman Billingham wrote:

The full OED defines pattress as "A wooden or plastic block attached to a
surface to receive a gas bracket, electric light switch, ceiling rose, etc.;
the base of a wall socket."

Dates back to 1886 according to the OED, when it was a block for mounting a
gas fitting. They quote 1969 A. J. COKER Electr. Wiring (ed. 7) v. 81
"Pattress boxes are also available to convert flush-type to surface
mounting"

The etymology it gives is interesting too:
"Probably an alteration of classical Latin pateras, plural of
PATERA n. (compare quot. 1905 at main sense)."

And the entry for PATERA is:
patera, n
Plural pateræ, pateras. Forms: 16- patera, 18 pattera.
[ classical Latin patera broad shallow bowl or dish,
perhaps the same Indo-European base as ancient Greek
{pi}{alpha}{tau}{gaacu}{nu}{eta} (see PATEN n.).

I've certainly always believed a pattress was surface mounting and I've
never used the term to mean a flush mounting box.

I agree entirely. I'd go further and say that the term should be
avoided altogether unless referring to the wooden blocks or moulded
mounting plates used in old wiring systems. For modern accessories
"flush mounting box" and "surface mounting box" are clear and unambiguous.

But then dictionaries reflect usage they don't create it.

Yes but... (... but we probably don't want the Queen of Hearts defining
technical vocabulary.)

--
Andy