While I'm applying the never ending layers of Tru-oil on my thinline ST build and having just completed my AG (well, mostly - but are they ever really completed?), I figured it's about time to start trying to source all the bits for this one.

So, I looked into the FR Special eBay link that Andy sent and noticed that even though it says R3 nut, the description says 10" radius but the R3 is supposed to be 12" radius. I figured I'd just ask the seller to confirm that it was a typo and it is indeed an R3 (12") nut since they have a separate eBay item with an R2 nut. I was expecting a reply that confirmed this assuming a copy/paste error but their reply had me less than confident about making the purchase. Something along the lines of there not being much difference between 10" and 12" radius.
Headed on over to Stewmac where they sell the FR Special kits and they're cheaper than the eBay auction, even with shipping factored in. However, the kits are only sold with R2 and if I want an R3, I'll need to buy that on top. Still, it's cheaper than buying all the bits separately. They also list the item as having a brass sustain block, but the picture looks like zinc alloy. They said they'll check and confirm that so I know if I need to order that or not.

So, a very long-winded way of getting around to my question. The sustain blocks are sold in 32mm, 37mm and 42mm. The 42mm is listed as being for flush mounted, 37mm for recessed and 32mm for deeply recessed. I'm assuming 42mm since I'll be flush mounting but to be sure, I'd like to measure something. Problem is, I don't know what measurement I should take. The 42mm seems like it should be the right one but I hate guessing.