Actually i'm sorry Alex, i didn't give you clear enough instruction before because 721 series are 'old-style' region-only units not accessible in Mass Storage Mode on a PC. Anyway, good news! The pack does have HWID 1667 V3.80 fw on it [i've also found an earlier one with V3.40 for hwid 1667 too].
The only downgrade instructions for this pack i've ever been able to find are from a Dutch site for echomap50, so i hope that's usable for you with the 721. I've translated to English and included it in an RTF file with a link to the original post in boote-forum.de in case you need it. You'll need to use a clean SD and make sure that a new xml file isn't generated before you've completed the downgrade. I'm uploading it all presently, but it's still 247MiB compressed and i've a slow adsl so check back here later. if you'd like to make a start reading up on the downgrade procedure, here's the thread link:
Post #39 has the instructions for echomap50 downgrade.
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EDIT: Here's the rar file:
Extract it to find the RTF file and ChartplottersUpdatewithSDcard_20140909.exe. Connect an empty SD card [minimum 1GB, maximum 8GB] and run the exe file which will create a garmin folder and place the gupdate.gcd in it. Follow the downgrade instructions from boote-forum.de and hopefully you'll end with V3.80 fw. Good luck.