As the specimens were very old, i am not able to set the wings, but i was able to pry them open lightly and true enough, Silvery has less black on the undersides because apparently the greater under primary coverts are grey, not black.
The greater under primary coverts of the Pied are white. In other words, if they're grey in Silvery, it has less white than the Pied, but no difference in the amount of black. Additionally, what can be hinted on the two photos I've been able to locate of Silvery suggests that its primary coverts to the underwing are white too (greyish in one bird - a shadow/light issue?). The primary coverts on upperwing of both species are black (as can also be confirmed by the two photos of Silvery), but in any case they're clearly also black on the individual on Mark's photos. If it isn't something caused by the angle, the individual on Mark's photos appears to have less black than a typical Pied on the remiges (specifically secondaries & inner primaries) or that they're proportionally shorter thereby revealing less black (the latter would be pretty odd, considering that the Silvery apparently was/is a small island species often travelling relatively large distances - something that really doesn't match short remiges well). Longer upper- and underwing coverts could be another explanation, but in the case of the former that doesn't appear to be the case for the Silvery based on the two photos, and in the case of the latter that would be unique in the entire family and is therefore highly unlikely. So, unless something can confirm that one of these scenarios is correct, we're left with two species with roughly the same amount of black to the wing - as indeed also suggested by all the literature I've checked.
In any case I (and I am sure others, too) would be interested in seeing the photos of the specimen Silvery. I had always been under the impression that most of the older "important" Singapore bird specimens had ended up in Naturalis (Leiden, the Netherlands) and especially NHM (Tring & London, England), but with at least 8 Silvery in Singapore that clearly wasn't the case.
Anyhow, having looked again, that yellow just seems too strong and - arguably more important - too restricted to just be explained by random sunlight. That combined with likelyhood (no records ever of Silvery on the specific island; no records anywhere for quite some time) leads to me still feeling quite confident residing in the Pied camp, even if I wish it was a Silvery.
Ding Li, a small hint that may help answer the questions you posted on the OBC yahoo group: The captive Silveries you've seen photos of were apparently kept in Hong Kong by a person named Crimson Lam. If you follow the below link (note: captive birds warning for people wishing to avoid that) there's an email address just below the page counter that you may try, but as the page itself apparently hasn't been updated since 2002, it may not work anymore:
http://www.geocities.com/itzalam/crimsonbird.html
The person specifically states that (s)he started keeping doves & pigeons in 1999, though another possibility is that the Silveries were kept by someone else and only photographed by Crimson Lam (actually, based on these and numerous other photos I've seen that were taken by this person, I get the feeling they're screen-caps from videos, but I could be wrong). Regardless, this suggests that this species has been kept in captivity at least within the last ~10 years.