Barfield DPS450 'cal mode' question

[ZZ]

I have a Barfield DPS450 air data test set I'm trying to cal. Using 33K6-4-3253-1, I can't enter the 'calibration mode'. The procedure is more centered around the 401/405/500, but the K-100 does list 3253 as the procedure. As per step 3.5.4 I'm supposed to press F1 and F4 simultaneously, but it won't enter the cal mode. Is there another sequence of keys to press to enter the cal mode?

Any advice on this one?