Not that it matters, but I think that may actually be a 17" frame. According to a 1989 Specialized catalog I found online, that bike was available in 15, 17, 19.5, 21.5 and 22.5.

I think my Rockhopper is a 19.5.



Ironically, I'm generally happy with 17" mountain bikes. For some reason this one feels smaller. I guess because they were still experimenting with MTB geometry back then.

You can probably get a Kalloy seatpost in the size you need for around $20 if your LBS doesn't charge you shipping.