Siam 1 Att. Surcharge On 64 Atts - Help To ID Pls.

Can anyone identify this variety? It doesn't look the same to me as any in my Scott catalogue, not least because it looks like there might be an extra letter above the final letter on the overprint (see detail).

I think I see 3 different things there.

There is a pale gray circular thing that seems to correspond in color to a marking next to the king's right eye (his right) that is sort of like a Thai "1" sideways. There is a similarly colored mark next to the king's other eye. Then there is a dark black marking that looks to be part of the postmark by its color. The normal overprint is in a gray black below all that.

So I think this is a normal Scott #67. Gibbons (for example) has this in two numbers (#46 and #47), 10mm or 8mm respectively.

Opinion:
I think merely the Thai script from the CDS (Circular date stamp) blending with the overprint, which reads

1 ATT
In Thai...Raka Neung Att (Price 1 Att)

I think the CDS is Krungthep (Bangkok)

Thanks for the responses. So it's Thai script from the datestamp?

Yes.
I can read "Krung" so presume Krungthep.
The date portion, I have no idea, it is too blurred for me to read.
I am not experienced in Thai datestamps, sorry.

If I was pushed, I'd suggest the date mixing with the overprint, is a "4" (or an "8")