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I know I'm just not looking in the right spot,but can someone tell me what years the BTV P-1 & P2 & P3 fit?
They are all interchangeable right?
Thanks again for all of your help an knowledge.BillW

My source (Wagner Lockheed service parts catalog, 1962) says:

P & P-1; 1952-54
P-2; 1955 (early)
P-3; 1955 (late)
P-4; late 1955 and 1956

I'll leave the question of interchangeability to those that have experience doing that.

In addition to O_D's info, There are 4 part numbers listed. 52 has the first, 53-54 the second, 55 the third which is labeled as a P3 in parts book and the 4th is for 56 which is not labeled in book but has a P4 stamped on my 56 unit.

They will physically interchange although there are some vent and vacuum hose connection differences in the early ones and between early and late. Apparently there was no specific changeover date other than early and late.

Hydraulic side appears all but identical but the major difference seems to be in the vacuum side for mode of operation and apparent response or feel. The first version was a sleeve valve type and the rest are poppet valve types. The later ones look to be identical so I surmise there must have been spring or orifice changes to improve something and that designates the P differences. Those improvements would probably be the main reason to use the latest.

See page 7.
https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?topic_id=6133&forum=4&post_id=66372#forumpost66372