At Traditio apostolica 41.5-6 we read:
And if indeed you are in your house, pray at the third hour and praise God. But if you are elsewhere and the occasion comes about, pray in your heart to God. For at that hour Christ was displayed nailed to the tree. For this reason also in the Old the Law prescribed that the shewbread should be offered at every hour as a type of the Body and Blood of Christ; and the slaughter of the speechless lamb is this, a type of the perfect lamb. For the shepherd is Christ, and also the bread which came down from heaven.
There is a variation in the Aksumite Ethiopic here. The text reads: “Pay careful attention to the time; for at that time we anticipate the return of Christ,” before going on to discuss the types of the shewbread and the lamb.
In any event it is hard to disentangle the logic here.
I have now come across the comments of Wenrich Slenczka, Heilsgeschichte und Liturgie: Studien zum Verhältnis von Heilsgeschichte und Heilsteilhabe anhand liturgischer und katechetischer Quellen des dritten und vierten Jahrhunderts (Arbeiten zur Kirchengeschichte 78; Berlin: de Gruyter, 2000) at 27-29 (a catchy title if every I saw one.) This I missed, so must admit to an error of omission in the second edition of my commentary.
Slenczka suggests that the verse regarding the shewbread is a gloss on 42B.3 (the following chapter)
This (the protecting power of God) Moses showed in the paschal sheep which was slaughtered. He sprinkled the blood on the threshold and anointed the doorposts, and showed forth that faith in the perfect sheep which is now in us.
This, it must be admitted, is possible though, as Slenczka admits, would have occurred early in the process of transmission. My problem with the suggestion is that, although I can see the connection between Moses’s lamb and Christ (not hard) the logic of the shewbread is less obvious, and the connection between the placarding of Christ (which is the connection with the shewbread) and the protecting power of the blood (which is the context for the mention of the lamb and its blood in chapter 42) creates a tension almost as difficult as the crux of interpretation that the movement of the verse is meant to solve.