Predicta - Tube type 21SFA - dangerous?

[madlabs]

Hi All,

I'm assessing the two Predictas I just picked up. On the inside of the cabinet of one of them it has a sticker which reads:

CAUTION TO USER
The picture tube type 21SFA is dangerous to handle. Refer all servicing and replacement to Philco qualified personnel only.

What's up with that? This one also has the sticker for having the 21EAP4 replaced with a 21FDPA.

Thanks!