There appears to be two types of this cancel but what is interesting is the date itself.
Your cover has the date...MY 12 30

while on the listed postmarks the day comes before the month.
This letter [internal] was most likely posted by the addressee, although I can find no mention of the name during searches.
The stamps are from the first issue of November 1929, printed by Bradbury Wilkinson in Lithography. Set of 5 although a 6 Puffin Blue exists from a rather strange and unexplained printing run. [Theories abound]
Nice cover. Most covers seen have the Lundy stamps on the back side of the envelope and it's not often one see's an internal 'mailing' !

Londonbus1