I reviewed these two injector assemblies and found only 1 difference: the Capsule Sub-Assy. Every other component has same part numbers. This tells me that is the only difference. I was not able to find just this part available for purchase.
Part Numbers Compared:
1990 1HD-T -> 23661-17010
1997 1HD-T -> 23661-17030
However, when I search the part number you referenced (23600-17010) for the complete injector, Amayama.com shows it as available for $246/injector. I am not saying it's guaranteed but it's worth taking a shot and based on quantity available figure I bet they have it. I have used them before. If they are NLA, you will get a refund.
Link to Amayama.com
Another source, Impex Japan, shows it available as well for about $188/injector. I have used them as well.
Link to Impex Japan.com
A 3rd source also shows it available for $391/injector.
Link to Mega Zip.net
As a final option the Denso part number for the 23600-17010 injector is 093500-4350 (please check).
Mud thread on this subject.
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