The CR250M fork staunchions (chrome sliders) are longer by around 2". Both are 35mm as previously mentioned.
It's a good XL250 mod if you want more travel on the front. Everything is interchangeable between the CR250M (73-74) and the XL250 K0 (72-73) forks.
Only other obvious difference is the external speedo cable mount on the RHS.
Best person to ask for exact length is forum member Nathan S , as I just sold him a set of CR250M forks and I do believe he has XL bits as well to measure against.