Refer to Moss Motors part number 151-500 Jewel, green, turn signal dash indicator lamp.
Part on left in picture was purchased and installed October 1986.
Part on right in picture was purchased for spare in September 1997.

Having partially stripped the thread on the earlier part, I was about to install the later spare in May of 2004 when I noticed the thread is the wrong size, and the newer jewel falls right through the threaded hole on the mating housing. (That's when I remembered why the part was not installed when purchased in 1997).

Early part has thread size 9x1 mm. (0.354" nominal) Later part has thread size 8x1 mm. (0.315" nominal)

Original mating parts:
Lamp housing (original) - has 9x1 mm female thread.
Washer, behind jewel - has 9 mm clearance hole.
MGA Dash panel - has 9 mm double-D clearance hole.

Below is a lamp kit, Moss Motors 142-400 from May 2004. Everything seen in the picture is included with this part number.

This is obviously a universal kit, intended for use in many different applications. In addition to three different colors of jewel, there is a grounding bracket with small lock washer and screw to be used when mounting the lamp in a non-conductive panel. Also included is a fiber washer which could be used to isolate the lamp housing (and grounding backet) from a conductive panel for an installation requiring two wires (like an ignition warning light). When used in the MGA only the metal shell and green jewel are used, in addition to the bulb and socket (originally part of the wiring harness).

What is NOT included in this kit is the 324-745 bright metal facia washer that goes between the front flange of the jewel and the front face of the dash panel. This part is shown in the catalog illustration as being included in the lamp assembly. However, the way the part description words are staggered in the listing columns, the trim washer (used special on the MGA) was not intended to be a part of this lamp assembly. I do remember many years ago having to buy the trim washer separately, some years after my restoration was otherwise finished, so that little bit of confusion has been around a long time.

I cannot address the authenticity of the small screw (for other applications), but it does work with the grounding bracket (although it is a VERY tight fit in the threads). The grounding bracket has a 9 mm double-D clearance hole, and the fiber washer has a 9 mm clearnace hole, both matching the 9 mm double-D clearnace hole in the MGA dash panel. The colored plastic jewels all have 8x1 mm male threads, and the (new) metal shell has 8x1 mm female threads. This means the parts in the lamp kit will screw together and can be used as a matched set. The mismatch of thread size to clearance holes means that the jewel may float slightly and might be misaligned up to 1/2 mm in the dash panel. This doesn't bother me, as I'm not a concours enthusiast, and I doubt that anyone would ever notice if you didn't mention it.

But what can definitely be noticeable is having the bright metal trim washer float off center by 1/2 :mm or more relative to the colored jewel. The OD of the trim washer is not much larger than the OD of the jewel. so a little misalignment sticks out like a sore thumb. If we have to live with non-original 8 mm threads, it might be nice to also have a smaller (non-original) 8 mm clearance hole in the trim washer.

What bothers me more (just a little more) is that the new colored jewels do not fit the original lamp shell, which has a 9x1 mm female thread. This means that for service parts the relatively inexpensive plastic jewel has to be replaced with the more expensive complete lamp assembly. Then if the jewel is ever produced in original form in future years, a correct new jewel would not fit this replacement lamp shell. Aside from the confusion and inconvenience any time the thread size changes, you might again have to buy a complete lamp assembly just to replace the plastic jewel. For now I'm satisfied that my car can be back on the road with a flashy green light in the dash, but it would be nice if these parts could be done up with the original 9 mm thread size (sometime).

Now another interesting observation. I notice the 1986 and 1997 vintage jewels are both rather dark green, which has always been a problem for light intensity. There has in the past been a tech tip floating around to counter drill a small hole from the back most of the way through the jewel to provide a small spot of brighter light in the center of the lense. I did this, and it doesn't help much. But all three colored jewels in the new lamp kit are much lighter in translucency. This means that the little green flashing light on my dash is MUCH BRIGHTER now, for which I am quite grateful, but I have no idea which shade of green might be closer to original issue.