how do you know which model Strida is which?

No problem, by downloading it appears in full size.
(Pictures bigger than 480 x 360 are not supported by the forum software).

Sorry to say that all Stridas lower than 5 are before my time,
maybe this manual might be helpful?
http://www.strida.co.uk/english/images/Strida_1_Manual.pdf

Already posted by Human_Amplifier here:
[url]Keeping Mk1 Strida riding for 40+ years]

I think yours is a Strida 2, please watch the locking mechanism and the part of the frame which is holding the handlebar, it is parallel to the surface, not pointing upwards.

Also your serial number is very low, please read here:
[url]Old stuff about the past generations of Strida]
and here:
[url]MK1 top plastic socket broken]