Tonga standard time changes, attn. Paul Eggert

[Alois Treindl]

attention Paul Eggert:
I am reading through the comments for zone Pacific/Tonga
and there you write on 2006-03-22:
... go with Mundell

where "Mundell" means his article by Don Mundell
http://www.tongatapu.net.to/tonga/homeland/timebegins.htm

In reading this article, I cannot see where he places the transition 
from standard meridian 185°e (12:20h) to 195°e (13:00h) into the year 1999.
What I read is that he talks about the change in New Zealand timezone 
from meridian 172.5°e (11:30h) to 180°e (12:00h) on 1946-jan-01, and 
that the Crown prince Tungi, wanted to maintain the 50 minutes 
difference to New Zealand. But it appears that no change was made then, 
in 1946, and Tonga now had only a 20 min difference to New Zealand.

I cannot see how you derive a change in 1999 from what Mundell writes.
Please clarify.

Shanks has this change in 1968, but as usual he gives no source.

PS: does anyone know of a way to get access to old IATA SSIM manuals? I 
rember those being voluminous books which travel agents used to look up 
flight schedules. For several years, I used to collect older copies of 
these manuals from a travel agent and use them to look up timezones, as 
they were documented for each airport, together with begin/end dates for 
DST if such a change fell into the two-month period a manual covered. I 
had a collection of many years from the late 1970ies and the 1980ies, 
but threw them out when I got hold of  Shanks' books. Now I would love 
to check some information in them again, but so far have been unable to 
find where copies are held in an archive.