Yishun was never under then-SBS control. It was then-TIBS first
ever area of responsibility, and was under them since the first bus
routes started to run there - and all the routes that were
originally started by TIBS over there (except Express 520) were in
the 8xx series.
Route 171 has always been part of SBS (until 1995 or 1996 (I
remember it as 1996, but SgWiki puts it at 1995), when then-TIBS
took over it) since at least it's inception and even before, if
what I can infer from the information I gathered is true. It
probably started life with Green Bus Company (GBC - no pun intended
with the shorthand form of GameBoy Colour 
) as GBC route 4, from Ban San Street (possibly
somewhere near where Queen Street is today) to the junction of
Mandai Road and Sembawang Road, according to the bus guides of 1966
. In 1972 and 1973, it was a United Bus Company
(UBC) route, and at that time it already has gained the number it
never shook off since - 171. Bus guides from 1972 and 1973
indicates that it was already operating out of Queen Street by that
time, but has an otherwise generally similar route to GBC's route 4
(still terminating (or was it looping?) at the Sembawang Road end
of Mandai Road. By 1979/1980, it finally came out onto Sembawang
Road and terminated near Nee Soon Camp (not too far from it's
original terminus, actually) it had also started to deviate into
the zoo via Mandai Lake Road at this point (this was where the
photo of the NAC MB O405 on route 171 on businfohub.com was taken).
However, it was still operating out of Queen Street at this point
in time, and (according to SgWiki) it was not until 1993 that it
changed course and went to Orchard Road and terminated at Marina
Centre (along with an extension into Bukit Panjang - how did it
enter Bukit Panjang before it was amended to BKE in 1999 by the
way?). By then, it was already going into Yishun, and according to
SgWiki, it was a Sembawang Air Base - Queen Street route at some
point before the Yishun extension in the 1980's. The introduction
of route 138 in 1994 made it skip the narrow stretch of Mandai Road
and the zoo completely. Coincidentally, it was the amendment of
another bus route that went to the zoo when it was with TIBS -
route 927 - that further changed it's route in 1999 - this time to
ply BKE between Bukit Panjang and Mandai Road.
I believe this makes route 171 the oldest route within SMRT's
control in terms of it's history (the oldest route that has always
been under SMRT (ex-TIBS) that never changed it's number is route
167, and route 167 and 980 shares this if you disregard
numbers).
