That's an interesting photo because the Maltby in the middle is wearing a Trials No4 forend. Notice the very straight bottom edge, as on the actual Trials No4 to the right.
That seems noteworthy to me, because if as we assume, the early Maltby and Savage No4(T) conversions were actually done at RSAF(E), not H&H, they would of course have had just those forends on hand, and if the original was found unsuitable, no doubt a replacement would have been fitted from those very same trials parts.
Of course it is conceivable that the forend was fitted by H&H or just happened to be fitted by an armourer, but the chances of that, especially the latter, seem pretty slim to me. We believe the left over trials parts were sent to Fazakerly to be used up in their learning process, but if the No.4(T) program was still in progress at RSAF(E) they might well have retained parts they thought they might need for that purpose: like forends.