There's a bit of a discussion about it in here somewhere:-

viewtopic.php?f=3&t=84060&start=30&hilit=chain+connecting+pin

But that basically answers the opposite of what I want to be sure about. (In there, people say 7800 chains cannot be joined with 7801 links)

My latest 7901 chain arrived with no pins, fortunately I had 1 (and only 1) spare from the previous chain, so i've used it up.

Just bought a pack of 5 spares ("Chain Connecting Pin - for 10 Speed, CN7801/CN-6600"). Are these identical to 7901 pins?

I don't mind wasting the cash I've spent, I do mind a snapped chain mid sprint!! Just want to be sure.

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Di2x2

Get yourself a Connex link to connect your chain. It's easy, allows you to easily take it off for cleaning and you won't have to worry about pin compatibility. I have been using them with 7800/7801/7900/7901 chains for the better part of 8 years and have never had a problem.

I have been using a kmc quick link with no issues on my 10 sp shimano Mtb chain. I know it's not the same as what you were asking but I bet it would work fine.

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TIME RXR

answering the actual question, from the 7801 generation forward, shimano 10sp chain pins have been the same. 7800 pins had one groove, 7801 pins went to three grooves to differentiate. oddly, sometimes you see them with two grooves, but basically it's the same.
this tech doc shows the two groove and three groove ones are interchangeable:
http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Chain/SI_09R0A/SI-09R0A-001-Eng_v1_m56577569830688497.pdf

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Di2x2