I would imagine that the point the DDE(trials) at Enfield were making (see BAR thread 34) was that even if you snipped the extending lugs off the H block, the original Thompson gun would still operate satisfactorily in a true blow-back mode. They must have produced a steel H block in order to ascertain that the delay was pure friction as opposed to increased friction between dissimilar materials. It even described the method, results and conclusions of the trial. With a bit of thought and adjustment, this is what the later M1A1icon turned out to replicate.