These look to be later iterations of what Thomas Henry Charles on page 5 around February 2021 posted, with David Halls' reply (ex- IAS) following shortly afterwards, (I was out of the loop on the thread at the time for reasons beyond my control). The differences with your model indicate the use of a SEAS midrange, (possibly as per the Beaulieu and the miniature 808 for bass/mid) and a Philips treble unit; the latter deploying the late Alan Willis' (ex- IAS and David's partner in the company and co-designer) perforated treble dome to address resonant peaks, as per the super little 808 quarter-wave horns found on this forum, (I have a pair and they are quite something when set-up suitably well). As for the bass unit, Thomas Henry Charles' bass unit looks to be an original, but what it actually is, as in manufacturer is baffling (no pun intended) as it has the trademark white painted cone, as was used on the earlier Studio 811 model of 1981 that I remember well at Essex Hi-fi when it was launched. The SEAS bass unit on your pair is not beyond a possibility as IAS used the famed bass/midrange unit from them as mentioned above, so obtaining an alternative wasn't beyond the realms of possibility. DomT made mention that he knew Alan Willis though, and so may well be right about it originally being a Volt driver being fitted to your version of the later 810 model. The only thing is that the earliest of Volt bass units had very low fs stats (free-air resonance frequency) with subsequent later generation units not being as low. From my work on my Beaulieu's from 1980, the very low fs contributes to a considerable difference in bass extension within the original David Hall/Alan Willis designed enclosures that had the full-width horn mouth at the bottom of the loudspeaker enclosures on the Beaulieu as compared to the IAS cabinets that were altered after David's departure from IAS in 1982 when another designer took his place and influenced certain changes that, to some listeners ears, were less impressive sounding, but maybe more measured. I cannot comment as I've not listened to any later versions. Included in the changes from the earlier models was the reversed internal layout of the horn-tract, which took the mouth of the horn to the top of the cabinet; probably to make the loudspeaker less overwhelming in reactive room acoustical environments by distancing the low-frequency output from floor-level. The mouth was also reduced in width. These changes were not only applied to the Beaulieu, but also, (it appears), to the 810 model too. Also, I think the earlier model was called the Studio 810, as the white baffle design was in-keeping with the Studio 811 model. Whether the original 810 model shared the 811 model's Medite cabinet, Thomas Henry Charles might be able to shed light upon this? David might be along to expand further on all this. I still find my Beaulieu's the dream loudspeaker system though, five years into owning my second-hand purchase from their original owner since 1980, and much as with your purchase, from Hampshire, near to their Southampton home.
I have only just come back onto this forum and can see more posts on our older loudspeaker designs. The design you have is using the Richard Allen 8" paper cone Bass unit. It also looks like an Eagle midrange which was used in the Cadnam Export. It uses the Horn Indederiminate length design that I developed, which was used in all of the speakers I designed with Alan Willis, my partner in IAS. I think this was orginally the Midhurst, which later developed into the 810. It is a long time ago and we were regularly bringing out various versions of our designs. They still look nice.