Mike
I have what you are looking for, PM me if you want images of the rest of the apparatus used with the flares.

The flares are given the following description:

HOLT FLARES.

These flares are used for illuminating the ground when landing at night. They are made by the Yorkshire Steel Co., Ltd., 30 Holborn, London, E. C.
The illuminating material is contained in a cartridge 4 inches long and 2 inches in diameter. Special hinger brackets shown in Plate 2, are provided, and are mounted on the under side of each lower wing. An elastic cord, which may be attached to a hook in front of the mounting of the bracket, serves to keep the bracket vertical when in use. When not in use, the cord is released and the free end of the bracket is tied back beneath the wing.

Each cartridge has two wires which are attached to binding posts on the bracket. Cables connect these binding posts to push buttons, two dry cells and a spark coil all mounted in the cockpit. A cartridge is ignited electrically when its respective button is pressed.

From AP1275, the description of the operation of the Mk III boost gauge is as follows:

The instrument—when connected to the induction system of an aeroplane—measures the pressure in that system relative to a standard atmospheric pressure at sea level, and at 15° C. Under altitude variation in flight the pressure will vary and this variation in pressure, as well as the boost pressure applied to the system by the supercharger, can be read on the gauge. It is a continuously reading instrument indicating the immediate pressure at all times. No other than visual attention is required.

KRA 0804K

I found it listed as fitted to RNZAF Sunderlands

From a RAAF instrument order on securing aircraft clocks to instrument panels (due to increasing loss of clocks from aircraft) the following is stated for American clocks (fitted with elastic stop nuts):

Sec/Ref P/N Description
G6P/8196 G.3982 Screws, special shear hd. No. 6-32 x 3/4

In an IPB that showed this clock being fitted, AN5743L1A (Elgin 1776), attaching parts are listed as AN526-632R6 or AN526-632-8

In other IPB’s the screws are listed as AN515-6R8, AN515B6R or AN507B632R16, with nuts AN365B632 or MS20365B632C.

Basically a 6-32 screw will do the trick, with a length dependent on the panel thickness and nuts being used.

What is the serial number of this instrument?

Surely stick on options is not the way to go, as if the compass is to be filled with alcohol this would affect adhesion.

The Section Reference number says it all, it’s the first item in the section (French aircraft gunsights and ancillary equipment).