https://gullfoss2.fcc.gov/prod/oet/cf/eas/reports/ViewExhibitReport.cfm?mode=Exhibits&RequestTimeout=500&calledFromFrame=N&application_id=245174&fcc_id='Q87-WRT54GR'

CPU: RDC R3211 SoC (not much info about it on the manufacturer's site, http://www.rdc.com.tw)
WLAN: Ralink RT2661 in a mini-PCI slot (MIMO XR, http://www.ralinktech.com/prod-2661.htm)
Switch: IC IP175A, 4 LAN, 1 WAN port.

Seems like serial and JTAG is there.

Products also based on this SoC: Gemtek WRTR-137G and Edimax BR-6216Mg, these have 2Mb flash and 16M memory, so this board might also have these values, hard to determine by the pictures. Interesting thing on the FCC photos is the WRTR-136G on the bottom of page 5.

Did not checked yet if it runs Linux or if there is a GPL tarball, will do so soon.

Here are the GPL tarballs. Looks like a prime candidate for OpenWRT.

ftp://ftp.linksys.com/opensourcecode/wr … _v1.01.tgz
ftp://ftp.linksys.com/opensourcecode/wr … RC_GPL.zip

Kaloz is currently working on an SMC device using the same hardware platform :-)