I looked through my L-E literature, but, failed to find anything on serial number 'blocks' for No. 5 Mk1 rifles.

However, Ian Skennerton, "The British Service Lee", page 179, states:

"Some changes which were introduced for the No. 5 rifle were the numbering of the fore-end to the rifle, approved on 14th November (1945), and the numbering of the magazine as well, approved on 3rd June 1946. It was also instructed that No. 4 rifles have their magazines numbered to the rifle.
Subsequently, all new production rifles from this date had the serial number engraved on the bottom of the magazine".

Whig's family heirloom, having been made in November, 1945, would not have had the serial number put on the magazine and probably not the stock forearm, (during manufacture).

Skennerton reports, (page 176-177), that an experimental metal 'nosecap' was fitted to the forearm tip, as early as July 1944, but, proved too difficult to manufacture.
A simpler version was devised and appeared on some 1945 production, but, was eventually discarded. Forearm tips were merely rounded off and the grain end sealed.

Skennerton does not give a date, that I can find, when the metal forearm cap again became a feature of No. 5 rifle production.