As predicted the 10 March Design Classics retail booklet (with the Phone box and Routemaster bus stamps) has 4 Machin 1st class gold with security features.
The booklet, shown above, is printed by Walsall Security Print. The printing is different, as you can see from the head - see below (sheet on left, booklet on right).
But comparison with the original sheet stamps also shows that the security slits are quite different - in the booklet there are four, rather than two. The Walsall slits are divided in two at the top and bottom.
These scans show coloured screening within the upright of the figure 1 - this is the security overlay.
Recycling in the RSFSR - Click on Images to Magnify Here is a March 1923 commercial registered cover from Petrograd to Berlin, the stamps cancelled with an oval railway station ...