The Hubbell, as well as the Pass & Seymour outlets (their top quality outlets) utilize Olin 688 Brass for the contacts, which is a brass alloy having approximately 75% copper content. The contacts are, in fact, this brass alloy and are not "brass plated". There is a lot at work with outlets contributing to the sound, possibly much that we don't even understand, and the P&S contacts are quite a bit different in terms of their shape than the Hubbells, which generally allow for more contact area on the blade of the male plug, at the expense of "grip", an area in which the Pass and Seymours excel. They sound quite a bit different and some will no doubt prefer the sound of the Pass and Seymours.
All hospital grades are NOT nickel plated. The Hubbell 8200H and 8300H (used in the Porter Ports and sold by Takefive) are not nickel plated and somewhat unusual in this regard as I mentioned above. The hospital grade Hubbell 8200 and 8300 (with no "H" designation) are in fact nickel plated and do have a different sonic character than the 8300H. The Hubbell 8200H and 8300H also have slightly larger (and it is very slight) contacts than the Hubbell 5262 or 5362 (which is also a very, very good receptacle and what I would expect Jim Weil to be using) and this may or may not contribute to what I perceive to be a slightly better performance from the 8200H/8300H when compared to the 5362.
For what it's worth, I have not heard any receptacle that didn't sound noticeably better when cryo treated, but the break-in process is ugly and they don't sound good at all immediately following cryo; hence my advice to just stick it on the fridge for a month or so.
There is a lot of misinformation and "voodoo" out there concerning outlets. I do not believe, for example, that any audio company has outlets that are really "custom" manufactured for them. When you take a good hard look at the "audiophile" outlets, the significant differences are essentially cosmetic (perhaps a logo on the front panel of the receptacle-PS Audio and Wattagate come to mind) and relating to plating of the contacts, which will obviously have an impact on sound quality. This does not, at least to me, constitute a "custom" outlet; it constitutes a stock outlet with extra plating added to the contacts. Very good quality outlets for audiophiles no longer cost a fortune if you know what you're looking for. That being said, I'd sure like to try out the Oyaides.