I recalled that there were three watch shops on the first level of Sunshine Plaza. Two were dealing in Panerai watches (new and pre-owned), and one was dealing in vintage watches of numerous brands - from Seiko to Citizen, ROLEX, Longines, Omega, etc.
If you are referring to the latter, I think the shop has shifted to The Bencoolen (beside OG/opposite Sim Lim Sq). Here is the (old) contact:
Name: HENDRY ANG
Company: Asia Time Piece Centre (A.T.C.)
Mobile: 91784921 (not sure if the number has changed)