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I've long since thought that GRX400, GRX600, 105 and Tiagra hyrdo brifters share essentially the same plastic lever body (bar the difference around the so easy to snap hose mount) and only the index mechanism differs. I already knew that the GRX400 series mech works with an 11 speed shifter (same pull ratio) but have not had the parts laying around to confirm the levers are essentially the same, till this weekend...
My daughters new to us CX bike came with GRX400 10 speed fitted while her old old has GRX600 11 speed. The better parts are going on the new larger frame but I thought it would be useful to keep the old frame built up for spares/turbo/pit usage and longer term her little sister. Laying around I also had a bust 105 shifter (that I cracked despite using the correct tool with a torque wrench). 10 minutes later we had success with just a swap of the index mechanism turning the 400 shifter up to 11.
Push the pin out the way then turn the bolt with circlip pliers.
Hmm these look similar... (10 speed on the left, 11 on right)
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The apparent difference is just a different gear being selected when I took them apart. It's very simply to put the mechanism back in, the single bolt requires just a few turns to do up and the retaining pins is very easy to move in or out.
So other than my case above where I now have 11 speed grx 400 why is this useful? Mainly because it allows for hydraulically broken 105 or GRX600 levers to be 'repaired' by swapping the index mechanism and lever blade onto the body from a 'cheaper' donor.
Nicely done. I'll have to file this away in my mind hole for if ever I need it.
Yeah it's a bit of an obscure 'need'. Might help someone though who wants to move to 11 speed cheaper (or to repair a lever with broken/cracked bleed port).