I was too tired tonight to complete my demonstration from Philo, 2 Corinthians and Clement's own writings that a κάλυμμα was a 'covering' not a curtain. I thought it might be useful however to note from the close-up here of Dura Europos images confirms our hypothesis about contemporary furnishing in the period. Rachel Hachlili (Ancient Jewish Art and Architecture p. 168) notes "the draped high back and the cushions of these types of thrones can be compared to Assyrian and Achaemenian reliefs; other details such as the turned legs and the method of draping the cloth are derived from Hellenistic furniture." Carl Kraeling (the Syngaogue p. 158) similarly concludes "the cushion of Esther's throne is blue, and the cloth covering the back yellow. With the throne goes a yellow footstool." For some reason none of the Jewish commentators I found take any interest in King Ahasuerus's throne which is similarly covered in a purple cloth.
I think it is safe to say that there can be no doubt that the practice was common place at the time Clement was writing and that 'the seven' things which 'cover' the truth are undoubtedly stromata draping the episcopal throne of St. Mark. Why would Clement have identified a throne with 'truth'? It is already established from him the Jewish prophetic writings "A throne will even be established in mercy, And a judge will sit on it in truth (aletheias) in the tent of David; Moreover, he will seek justice and be prompt in righteousness." [Isaiah 16:5LXX] Indeed the idea of the throne of truth being established in the 'tent' of David is undoubtedly where the κάλυμμα association comes from (as Philo and Clement understand the 'outer tabernacle' to be a κάλυμμα).
The only question I have left is whether the canopy that is often suspended above thrones is ever called a κάλυμμα ...
The answer, it turns out, is affirmative - κάλλυμα σκιάς κλίνης, κάλυμμα σκιάς θρόνου. All of which raises the other possibility that the inner sanctum was 'covered' by seven stromata. This is certainly the sense of passage originally in Philo and then reused by Clement. First the reference from Clement in the fifth Stroma:
The covering (κάλλυμα), then, the barrier of popular unbelief, was stretched in front of the five pillars, keeping back those in the surrounding space.
And now Philo, the original source where Clement drew this understanding:
And proceeding onwards from thence he [Moses] wove the tent of the tabernacle with two boundaries of space, placing a covering (κάλλυμα) between the two, in order to separate what is within from what is without. And also he gilded the sacred ark, the place wherein the laws were kept, both within and without; and he gave the great high priest two robes, the inner one made of linen, and the other one beautifully embroidered, with one robe reaching to the feet. For these and such things as these are symbols of the soul which in its inner parts shows itself pure towards God, and in its exterior parts shows itself without reproach in reference to the world which is perceptible to the outward senses and to this life: with great felicity (On the Change of Names 43)Morton Smith's understanding of the passage can be disregarded. There are not seven 'veils' which stand between the initiate and the inner sanctum as a curtain but instead 'coverings' which either literally sit on top of a piece of furniture (the throne) or perhaps literally 'cover' the whole inner sanctum one after the other as a clear symbolic representation of the seven heavens. I will have to sleep on this one but welcome any input from readers.