Gerben,
Could it be possible that someone has reblued the sideplate with instant blue?
The changing from blue to brown is a common failure when the instant blue is improperly applied, especially over a long time period.
ViggoG
Edit Addition; <img border="0" alt="[blabla]" title="" src="graemlins/a_smil17.gif" /> <img border="0" alt="[blabla]" title="" src="graemlins/a_smil17.gif" />
Gerben,
The excellent book by Angier, "Firearm Blueing and Browning" covers the subject in fair detail.
They are occasionally available on "eBay" at a very reasonable price.
I highly recommend it as a referrence work.
<img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />
ViggoG