While it’s a fantastic song all in all, for me it’s all about the drum track. It’s so unfussy, so methodical and regular, and then so satisfying when the drums break out into a quick rattle at the close of some of the vocal lines. I’ve not been able to find out for sure who the drummer is, but Motown’s house band The Funk Brothers are usually credited so the likely candidate is Richard ‘Pistol’ Allen.
It’s amazing how much he’s able to achieve in the moments that he allows himself to escape from the standard beat, and I love the way that towards the end of the song he lets the rat-a-tatting take over little by little, threatening to transform the song from Stax-esque funk into a wild marching band.
Click here to listen to Signed, Sealed, Delivered I’m Yours on Spotify.
Tim Major 
“the rat-a-tatting take over little by little, threatening to transform the song from Stax-esque funk into a wild marching band.”
is a perfect description, transforming a good song with a great hook into something insanely funky.
http://soulfuldetroit.com/archives/624/370.html?1020896034
for a discussion on this matter with some of those involved in the recording.
Amazing link to some of the people who worked on the original!
Baffled by the claim that there are two drum tracks overlaid( Pistol and Stevie himself); it’s so raw and stripped-back, how can that be?
My own hunch had been that it was Stevie’s own drumming, and that it was virtually a demo track; the fills sound improvised and a little ragged to me, but so vibrant and on the money there was no point to try and improve on them!
Couldn’t agree more, drums on the whole album are incredible!
I think the original drummer (uncredited) was Billy Cobham. That’s why so good.
Really? Well, that more than explains it, then!
There are definitely two drum tracks overlaid on the song. The more spastic is undoubtedly Stevie. The more solid one of the Motown in house drummers. Stevie isn’t exactly the most rock solid drummer ever. On his later albums where he plays all th instruments you get to hear his voice on the instrument and its quite different than the usual Motown sound.