NavList:

The market, at least on eBay, seems to agree with you that there are few differences. There are no cheap GLH- sextants any more. Almost all seem to be described as Astra’s or Astra/GLH- and are all over $400.

I looked at 21 examples for sale, or sold, trying unsuccessfully to spot any consistent differences.

The company names on the papers have lots of variations of Celestaire, Changzhou, China, Marine, Haiyue, Instrument, Equipment, and Technology.

Serial numbers on the papers or on the sextant range from four digits to seven. There was a 1989 with a six digit SN and a 1998 with a four digit SN. All with dates after 2009 had seven digits.

Dates on the papers range from 1988 to 2017 - no significance there.

I was surprised to see two separate ads that had photos of two very different serial numbers on the sextants in the same ad.

You will find plenty of GLH130-40/20 on alibaba.com and made-in-china.com, with advertised prices starting at US$ 280 for a GLH130-40. As the eventual terms are individually negotiated on these platforms, I don't know how prices would work out in practice, lacking experience with these platforms myself. I have not seen any Chinese sextants marketed on these platforms as "Astra" for some time (but maybe I did not look hard enough). The advertised product images that I have seen all show the round index mirror of the generic Chinese GLH130 rather than the square one of the more recent "Astra" versions of the GLH130. As with other technical products made in China, it is not unlikely that all GLH130 sextants are made at the same factory and then branded according to the respective reseller's specifications, by adding the name sticker on the arc. The company names on the papers might also reflect different resellers rather than different producers. I, too, have not been able to determine any pattern for the serial numbers across differently branded GLH130s and tend to think that each reseller, including Cassens & Plath for their GLH130-40 branded as "CP Sailing," gets to to specificy their own sequence of serial numbers (in contrast to Cassen & Plath's own sextants, which kept their original numbering when they were occasionally sold under other names in the past).