Mystery Instrument Dial

I recently purchased an unusual ‘Instrument’.

It is marked on the face as a standard Mk1B RoC would be except for the absence of a Sec/Ref number. The face is marked 0-4,000 fpm and UP with the following etched:
‘MARK 1B
No 9520/41′

The rear of the case is marked as follows:

SUB-STD
INST.DIAL
SERIAL No 24
0.100 c.f.c. (crossed out)

0.080 e.f.c. 2-50 (crossed out)

0.105 e.f.c 1-51

There is no mechanism in the case, the case only being 1.25″ deep, only a mounted face.

I am assuming that the idea for this type of dial is as a standard or sub-standard against which others are checked, and either passed or failed. A rudimentary form of quality control. I suppose things had to be performed at quite a pace during the inspection phase of manufacture. I have a Smiths Aircraft Instruments Catalogue stating that Smiths manufactured 10 MILLION aircraft instruments and mechanisms during the war.

What also makes this interesting is that when I took it apart to clean/paint the RoC face is affixed to a Mk1A Turn and Slip Indicator Face complete with centre stating the ident of the instrument.

I also have a similar styled ‘instrument’ in the form of a Mk1B Artificial Horizon, this also has the ‘instrument’ face attached to a turn and slip face! I have no pictures of this at the moment though.

I have attached some pictures of the item, and just wondered if anybody had some further information on these. My dad who has been in and around aircraft spares since the age of about 12 (not 70+!) has never seen anything similar before.

TIA
Regards

Dave