There is a question as to exactly which OD unit was used in 55-56 -- if R11 or maybe R12 and what are all the differences. One for sure is no lockout switch in 55-6. I can't say if solenoids are the same construction. Probably so, but am sure others can say for sure. One thing to watch for is solenoid travel length. Once you find the trans/OD unit to install you can tell. Others have made the conversion so surely not a big deal finding one.
One place you might check for a solenoid is 5th avenue garage
http://www.fifthaveinternetgarage.com/ They have repro OD items for standard BW units which might be easier to chase down than a 55-6 item. Their solenoids are not inexpensive but then neither are Packard vendors. Making sure theirs has the correct travel length is the issue.
Also have repro relays but as you said, those can be substituted quite nicely with a Bosch 30 or 40 amp headlight type relay from the parts store. Just won't have the metal can and fuse looks if that matters.
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