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Dammit-
Cheap one here :-
https://www.ebay.co.uk/itm/124655827651?hash=item1d0610fac3:g:ZkoAAOSwmCxgX4Ct
The Parf one is just for people who want to use dogs and need them perfectly lined up.
Airhead
There's also the Parf guide (named after Peter Parfitt)
https://www.youtube.com/watch?v=Bfxxjn85SRE&ab_channel=AxminsterTools