{Collins} re 75A4 filter

To all: I recently bought a 75A4 with three filters. One of them is a Collins 300 hz xtal filter, p/n 526 7073 009, mounted on a silver plated chassis with a 9 pin plug that really looks like Collins quality. It plugs directly into one of the filter sockets. I worked for Collins here in Toronto for 15 yrs but have never seen or heard of this filter. Anyone out there recognize it?  I would sell it for $200 if anyone is interested. It works great for a cw buff.
Tnx  Jim  VE#3DSR

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