Production of the No5 started in Feb 1944 with the FE prefixed 1000 & the normal production began shortly aftrer in March /April, i have a 4/44 with the serial number 524 with a un-numbered forend & mag, no cap on forend.
According to the "Bible" in May 1944 a request was made for a cap to be designed to fit the end of the forend & in July 1944 the rounded type was fitted to an experimental rifle & also to some experimental lightened No1s but was found to "difficult to produce" so was changed in September of 1944, there is no mention of this early cap being fitted to production No5s but a photo of one fitted is shown in the book.
No date is given for the later cap of the type we see but Ian states the rounded cap-less forend appears to be preferred type.
Nothing really definitive in that but we can see, no cap, cap, no cap in 1944 production.
When studying the No5 a trend appears showing that the cap appears in earnest in mid 1946 production & as the numbering of the forend was not approved untill Nov 1946 the fitting of salvaged early forends to later No5s & being numbered to the rifle & the fitting of new forends with the cap to early No5s, plus the removal of caps in-service all serve to grey the whole thing but if you look at the No5 survey on SRF you can see the trend.
All in all, not an absolute & at the end of the day does not really prove anything, but its all we have to work with.
Surplusrifle Forum View topic - No5 Mk1 Jungle Carbine S/N Survey! - Updated 11.2.2013