La2a Valve Screening Cans And Valve Bases?

Looking at a picture of an original La2a i notice all the valves have screening cans. In my junk boxes i have screening cans that just slide over the valve and grip the glass body and there are the types that have a twist type of fixing that mate with a shroud mounted on the valve base. Do i need to buy valve bases that have a screening can fixture mounted on them or is the fixture bit available separately and then fixed along with the valve base, allowing the screening can to twist fit onto the base :?
Vikki(uk)

If your always moving cables around behind the rack, you might want the locking shields. This is a line level instrument, so shields are not that important for noise, in fact, some people take them off on their guitar amps as it is said to improve tone. Capacitance to ground, disruption of the electron beam, etc. You won't find any tube shields inside a Tele V76 mic pre.