It's tough because even the technical specs on older Brunswick Balke-Collender pool table brochures don't specify the cushion profiles (but rather, usually just the brand e.g. Monarch).
But yeah, I've looked into this before for others asking the same question, and I've also been told they were K-55 cushions - BUT - there's a big point of confusion—which can only be resolved by taking some measurements of your rails (see the blockquote at the very bottom of this answer).
Brunswick Balke-Collender's rail taper changed slightly over time as their cushion brands changed.
The older Brunswick Monarch Superspeed cushions aren't the same as the modern Brunswick Superspeed cushions. More meticulous pool table technicians will want to modify your rails to accept modern cushion profiles.
Many say the modern K-66 is "close enough" and will work fine.
Here's a post from another site by a pool table technician:
This is my personal experience with this issue. Modern K-66 cushion measures the closest to the original monarch cushions but do to the fact that the geometry of the meat of the monarchs and modern day k66 cushions are different. K-55's have close to the exact geometry of the original monarchs but measure very differently when comparing to a ruler.
For example, K66's, if mounted flush to the top of a unmodified rail will put the nose of the cushion high, where as k55 will put the nose of the cushion at the right height, but you lose the 50X100 playing area on a 9 ft table. I put K55's on old brunswicks if I have to just replace cushions and the customer dont want to spend the extra money to have the rails calibrated for a perfect playing field. If I ever get a customer who wants a true playing area I will pass the rails down to a Master mechanic here in southern california (SDbilliards) to calibrate them.
One benefit of just replacing Monarchs with K-55's is you gain a deeper pocket shelf, but you sacrifice the true playing field.
Its really hard to explain the geometry of the meat of the cushion and the angle in relation to the subrail angle. If you take a slice of the monarch cushion and a slice of the k66 and k55 and stack them you will see that the K55 follows a similar profile.
Now consider this blurb below, from Classic Billiards (owned by Ken Hash, an expert pool table restoration guy who is very well informed and well respected):
K55 cushions measure 1 ¼” across the top and 1 5/16” high at back (glue) side.
Keep in mind if you have an older Brunswick (prior to 1975), be sure to measure the back side of the cushion at the glue side and confirm 1 5/16” before ordering. Older K55 cushions had a 1 5/16” back side dimension but a 1 1/8” top side measurement (same as K66).
If your table is an older 7- or 8-foot Brunswick table and it has the Brunswick Centennial or Contender cushions (non triangular cushions, shaped like a “number 7”), then you need to order Brunswick Contender cushions. We also sell our Classic Billiards U56 profile cushion as an great affordable alternative.
If you are not getting an existing nose height for your old cushion in the range listed above, then call us at (410) 256-8388 and we will help you determine correct profile needed.
Also if you are getting a cushion dimension different than the K66, K55, and U56 profiles listed above with your contact point being correct at 1 3/8”- 1 7/16” above playfield, then you may have a smaller or uncommon profile. Feel free to call (410) 256-8388 and we can try to match up to a correct, current cushion profile.
If there's any questions, I would call these guys. Ken Hash (the owner and a master pool table restoration specialist) is super helpful and always happy to help. We're not connected in any way, but he's the guy I usually recommend for this stuff because he's recognized in the industry as being very knowledgeable and willing to help.